Close this search box.

Underground Adventures In Spain: Cave Projects


Ario Caves
Ario Caves

Cave diver Chris Jewell heads to the Picos de Europa in Spain to take part in two very-different projects, beginning with the ambitious and challenging Ario Caves Project.

Photographs by Chris Jewell, Mark Burkey and Bartek Biela.

Location and Overview

The Picos de Europa in Northern Spain is a 20km-long mountain range with peaks reaching over 2,600 metres. The mountains are mostly comprised of limestone which means they contain many caves. Where there are caves, there is usually water, and for me that means cave diving exploration. This summer presented me with the change to take part in two significant cave diving expeditions in the Picos.

Ario Caves Project

The first objective was a sump (water-filled passage) at the bottom of a 900 metre deep cave called Cabeza Muxa, which was located high up on the Ario plateau (1,630 metres altitude) in the western massif of the Picos. That meant that in order to dive the sump, we needed to get everything up a mountain, then down a vertical distance of 900 metres, as well as along a 2km cave passage following a fast-flowing river. Oh… and we also needed to install all the ropes (totalling about 1,300 metres) along with new rock anchors (150 stainless steel bolts) and set-up an underground camp! You can tell I like a challenge…

Cabeza Muxa was last visited by a UK team in 1988, when the downstream sump was dived by Rick Stanton to a depth of 33m using open-circuit scuba gear. It has remained unexplored since then and offers the potential for a significant hydrological connection to other caves in the area.

Preparation and Logistics

The team discussing the forthcoming dive
The team discussing the forthcoming dive

Given the logistical challenges involved in this project, I was going to have to make some some smart choices about equipment. Firstly, I own some nine-litre carbon composite cylinders which weigh about 9kg out of the water. The problem, of course, is that these require around 6kg of weight to make them negatively buoyant.

That weight would need to be carried down the cave, but at least it could be separated from the cylinders and then staged for future expeditions. The weight of everything else would also need to be examined and reduced where possible. At the same time, I’d need sufficient spares and tools to deal with most dive kit eventualities.

Finally, everything would need to be dismantled and packed in robust bags and containers for the long trip down the cave.

Arrival and Initial Expedition

Helicopter to deliver supplies
Helicopter to deliver supplies

When I first decided that it would make a good objective for summer 2022, I knew that it would be hard work. We’d need a strong team and anything we could do to make it easier would really help. I couldn’t do much about the cave, but we could get a helicopter to fly the gear into base camp at the mountain ‘refugio’. After the long drive down through France with a fully loaded van, we arrived in the Picos. The next day we drove up into the mountains as far as the track would take us. From here a helicopter did the rest of the work, lifting 900kg of kit into the sky and off to the refugio while we made the two-hour hike up.

Caving Begins

Prepping for the dive
Prepping for the dive

The caving began straight away with a sense of urgency and purpose. After five days of hard tiring caving, including three nights where we camped underground, we had the cave ‘rigged’ to the sump so that transporting diving kit was possible. Two days later re-enforcements had arrived on the expedition so a team of six fresh cavers entered the cave with heavy bags of diving equipment each. Then the following day myself and three others (Lisa Wooton, Stu Weston and Mark Burkey) went underground for the main event – to dive the sump.

Dive Preparation and Unexpected Events

After descending the 600-metre-deep shaft series we reached underground camp before heading off along the 2km stream way which drops 300 metres via 26 roped pitches to the sump, where I would dive. Unfortunately, just over halfway down this section of cave, disaster struck. Our expedition photographer Mark Burkey was approaching one of the roped drops when the rock he was holding on to broke. Mark fell forward on to his face, with his nose taking the impact.

A lot of blood followed, and it was clear he’d broken his nose. Everyone fully expected the trip to be turned then and there, but Mark was determined that the dive would go ahead and that he’d be there to document it.

Several hours later we arrived at the sump. I prepped my dive gear aided by Stu and Lisa while Mark prepared to photograph the effort. However, when he unpacked his camera he discovered to his horror that his fall had damaged the case and caused it to leak. The precious camera was flooded, and no more photos would be possible!

Dive Execution and Equipment

I expected the dive to be deep and possibly lengthy, so in the chilly 6 degree C water I would certainly be using a drysuit and it also made sense to take a rebreather for this project.

I’ve been diving a KISS Sidewinder for a couple of years with good results and so that was the obvious choice. Given the potential for depth, I elected to take a nine-litre cylinder of trimix as my offboard diluent and deep bailout plus another nine litre of EANx 30 to feed my drysuit and for shallow bailout. A two-litre bottle of oxygen to run the rebreather and a seven-litre bottle of oxygen staged at 6m depth in the sump completed the setup.

In most cave diving I do, I almost always use a large cylinder as offboard diluent and bailout combined, then a second cylinder acting as suit/ wing inflation and further bailout. As a streamlined rebreather, the KISS Sidewinder tends to be dived like this. By contrast, most other conventional rebreather configurations and therefore training,
separate diluent from bailout gas, while separate suit inflation cylinders are also very popular.

Did you know?

The Picos de Europa National Park contains meadows, lakes, mountains such as the Naranjo de Bulnes, gorges and impressive forests which host large mammals like rod deer, along with grouse and Egyptian vulture.

For me there are several advantages to combining the cylinders. Primarily it allows me to take fewer cylinders into the cave. Most onboard diluent or suit inflation cylinders will be small, making them suitable for one single dive. If cylinders are being carried a long way into a cave, then a single large cylinder being used for multiple purposes makes sense.

Similarly, I would rather dive with two large cylinders than several smaller ones, as it makes kitting up and diving much more comfortable and streamlined. These advantages, of course, need to be balanced against the downsides. A mix suitable for diluent and bailout has to be selected, there is less redundancy in the system (partially mitigated with the inclusion of a Y-valve) and gas safety calculations need to factor in that bailout gas is being consumed as diluent or for buoyancy.

Once these considerations are understood then hose routing and a system for plugging in the offboard diluent needs to be worked out. Ideally a single offboard connection to the rebreather should feed both the ADV and the MAV simultaneously. Many rebreathers boast the capability to plug in offboard diluent, but very few do so without needing the diver to disable the ADV and only add diluent manually.

Underwater Exploration

While the team kept warm in a storm shelter making hot drinks, I entered the water and began my underwater exploration. The excellent visibility that I had expected was reduced by the sediment that my kitting up had disturbed, however, I could still easily see a good five metres. At 6m depth, I dropped off the bailout oxygen and began spooling out dive line.

The 2.5mm thin white cord was marked every ten metres with a small piece of yellow tape featuring a handwritten distance number. I’d carefully prepared the line weeks ago at home and wound all 600 metres of line on to one of my ‘homemade’ line reels. As I progressed further into the cave, the visibility improved. Large calcite mineral deposits hung to the walls in lumps and with little else to attach the dive line to, I wrapped the string around these, sending little puffs of sediment into the water.

The winter floods in the Picos are known to be ferocious and I’d not expected to find Rick’s line from 1988 in good condition. Preferably the line would have been completely washed away, but instead the floor and walls of the sump were strewn with old dive line, creating a hazard if I was unwary. In places I cut the old line free and elsewhere dodged around it.

At 30m depth I pushed into new territory. Visibility got better again but the way on wasn’t obvious as the passage twisted and turned, going up then back down again. There were now precious few places to belay the line in the clean, washed passage. After about 150 metres total distance, the sump then began to trend upwards.

After passing under an arch I found myself at 15m depth in the bottom of a steeply ascending shaft. With nothing to secure the line to, I spooled out while rising upwards. At 8m depth, the reflective surface above became visible. Taking it nice and slow, I reached the surface where a never-seen-before tall slim passage led away from a circular sump pool.

Discovery and Challenges

I’ve been lucky enough to successfully pass sumps and find dry passage on a number of occasions, but the thrill is still just as great. There is excitement at the unknown and relief at the respite a dry chamber represents, all tinged with the apprehension and doubt about the return journey.

My first priority was to secure the dive line which represented my safe route home. Then after dekitting, I headed along the newly discovered cave passage. However only 15 metres from the sump pool, the next obstacle was found. A short vertical drop with the whole streamway crashing down filled the passage. Although the drop wasn’t more than two metres, the floor and walls were smooth and slippery with a calcite deposit and there were absolutely no foot or hand holds.

I was diving solo, something I’m very used to doing. In this case the rationale was driven by simple necessity. Portering diving equipment for one diver would be difficult enough, double the equipment wasn’t feasible given the time constraints. As a solo diver it’s important to be completely self-sufficient and all of your equipment, configuration and gas planning choices take this into consideration. While this made sense underwater, back in the dry cave being on my own was much less ideal.

If I climbed down the drop and couldn’t get back up, I would be waiting a very long time for rescue without anyone to help me up. There were other divers on the expedition but there was no other diving kit in the cave and it would be many days before anyone could come to look for me. Knowing there was no safe way down and that the risks couldn’t be justified, I turned back.

Returning and Surveying

Now I had one final but vital task – to survey the new cave passage I had found. Although new technology has introduced more options of cave mapping, the basic principle remains unchanged. To produce an underwater survey requires measuring the distance between two points connected by a straight line. At each point the diver needs to record the depth and the compass bearing to the next point. For manual surveying, a well-tagged dive line provides the distance between the two points, a compass the bearing and dive computer the depth.

With this data entered into a surveying programme, trigonometry calculations can provide a map of the underwater passage. So, swimming slowly back through the sump I took careful measurements at each belay and every corner where the line changed direction. Back on the surface, my scrawled wet notes and the video of the dive were now my most-precious possessions! It had taken many months of planning and organising as well as the physical effort to enable this dive. Now the results of all that effort and the entire record of my exploration was contained on some sheets of paper and a memory card.

Exhaustion and Exit

While I’d been diving, the rest of the team had kept warm and well fed but everyone was getting tired at the end of a long day. After packing everything away, the team finally left the bottom of the cave at midnight. A very, very slow return to camp with heavy bags was made, with myself and Mark reaching camp at 4.30am, and Stu and Lisa at 6.30am. The following day no one wanted to get up early, but by midday we were awake and eating breakfast in our sleeping bags.

The underground radio link we had with the surface meant we knew that that four cavers were coming in and so we planned our exit. Many slow hours of prusikking up ropes later, the surface was reached. Food and, of course, a well-deserved beer was not far away!

Exploring Tresviso: The Village above the Urdon Gorge

Tresviso basecamp
Tresviso basecamp

For the final part of my time in Spain, I joined the Tresviso Caves Project. Operating from the mountain village of Tresviso (Cantabria) in the eastern massif of the Picos de Europa, the current expedition team has been mapping and exploring caves in the area since 2015. However, cave exploration here has been taking place since the early 1970s, when Lancaster University Speleological Society (LUSS) first visited the area.

Tresviso is situated above the Urdon Gorge, where a major cave, Cueva del Nacimiento (also known as Cueva del Agua), resurges. The water which leaves the cave is captured by the Canal de Urdón, which takes it to the nearby hydroelectric station. Each day, millions of litres of water flow out of the cave, which is indication of how significant it is for the

First Dive: Journey to Rio Chico

Downstream from Cueva del Nacimiento is a second resurgence cave, Rio Chico. This site was explored in 1986 by Steve Jones, who reached a depth of 62m with the passage still going down. Fresh from my earlier exploits in Spain, we wasted no time and on day one, a team was assembled to carry diving kit down into the gorge. hydrology of the limestone mountains which tower above the gorge.

To reach Chico, it is necessary to walk in the Canal de Urdón for a short distance and fortunately in early September, the water level was still low enough for us to get to the cave. It was my first visit to the site and I didn’t know what to expect, but thankfully it was a fairly short carry to the sump pool, which we reached after about 15 minutes.

As with my earlier expedition, once again I was diving a drysuit and Sidewinder rebreather. Due to somewhat easier access and deeper diving, my bailout and off-board diluent plus suit gas were a pair of 12-litre cylinders, one with a Y-valve containing a TMx mix and the other with air. A seven-litre bottle of o2 was also carried and staged at 6m in the sump.

The entrance to sump one is a good dive base to kit up at and I was ready quite quickly. Sump one was passed following the installed dive line and I then staggered fully kitted into sump two, up a very small cascade.

The visibility was a disappointing four metres in the large sump. Following the original thick blue 4mm polyprop line installed in 1986, I descended quickly to 45m depth where the old reel was encountered. The old line was in good condition and the reel looked serviceable, so I picked this up and carried on descending. As I used up the last of the line, the floor came into view at a depth of 80m. A suitable rock was located to tie off to and after consulting my dive computer, I decided I had enough time to carry on exploring.

I attached my own reel here and ran out 90 metres of thin new line in a large passage which was clean washed with very few belays.

At first, the passage trended upwards and back to 65m depth before dropping steadily back down. By the time I’d reached 79m depth, I was ready to turn back and my NERD dive computer was telling me I’d have a long decompression on the way back.

After searching for a final belay to secure the line, I cut the reel free and turned for home. I wanted to head back quickly, mindful that every extra minute at this depth was costing me a lot of decompression time, but I also knew that I needed to come back with some survey data. Reluctantly, I pulled a set of wetnotes out of my pocket and began the steady return dive, scribbling down readings as I went.

The Ascent and Post-Dive Reflections

Tresviso Caves Project
Tresviso Caves Project

With very little horizontal passage I had little chance to off gas while moving. That meant that all my deco took place in the deep shaft while circling the line. A familiar experience to deep wreck divers, but in cave diving I’m used to having somewhere to go. In total, I completed approximately two hours of decompression with about an hour at 6m and a total dive time of three hours. Back on the surface most of the team, except two, had left the cave.

With their assistance the equipment was pulled out of the water and stashed on dry ledges for recovering the next day. I was very aware that I had a very big hill to walk up (400 metres of ascent) and so I took things very slowly. I dekitted and we all made a very slow ascent of the hill back to Tresviso, my precious survey notes safely in my rucksack.

With very little horizontal passage I had little chance to off gas while moving. That meant that all my deco took place in the deep shaft while circling the line. A familiar experience to deep wreck divers, but in cave diving I’m used to having somewhere to go. In total, I completed approximately two hours of decompression with about an hour at 6m and a total dive time of three hours. Back on the surface most of the team, except two, had left the cave.

With their assistance the equipment was pulled out of the water and stashed on dry ledges for recovering the next day. I was very aware that I had a very big hill to walk up (400 metres of ascent) and so I took things very slowly. I dekitted and we all made a very slow ascent of the hill back to Tresviso, my precious survey notes safely in my rucksack.

New Objective: Diving into Nacimiento (Cueva del Agua)

Although my dive in Chico had been very successful, the poor visibility was slowing down exploration. Also, the increasing depth meant additional cylinders and equipment and, as a solo diver, this wasn’t ideal. So I decided to turn my attention to the other objective, sumps in Nacimiento (Cueva del Agua) itself.

‘Certain Death': Venturing into the Unexplored

Kitting up at Certain Death Sump
Kitting up at Certain Death Sump

In 1976, the cavers exploring Nacimiento located an unusual feature. After Colin Boothroyd scaled an eightmetre waterfall, they found a deep sump pool immediately at the top of the climb. Inspired by Monty Python, they named the cave passage the ‘road to certain death’ and the sump itself became ‘certain death’.

Rob Parker was first to venture into the water in 1985 and again in 1986 when he reached 64m and reported an ascending shaft leading on. Gavin Newman then took up the challenge of diving here, which culminated in the 1996 trip, where he made a film for the BBC’s Extreme Lives series about diving the sump with Phil Short. The film was called ‘The Road to Certain Death’.

Since 1996, the sump has remained undived until this year, when I was able to make an attempt to explore it thanks to an excellent team of cavers from the Tresviso Caves Project. The sump is located approximately one hour from the entrance, further than the trip into Chico, but thankfully nothing compared to the depths of Cabeza Muxa. Before I could dive, however, we needed to re-climb the seven-metre waterfall and then install ropes so that we could access the sump.

Next, the heavy and bulky diving equipment was carried in for me to dive. I dressed at the bottom of the waterfall while the team hauled the kit up to the lip of the pool. Then, in my drysuit, I donned a caving harness and climbed the rope. At the top there was very little dry land. Balancing on the precarious ledge, I was able to put on the rebreather and then while treading water in the deep pool, I attached my cylinders.

The original dive line, thick polypropelene, led off from the pool and I followed this down into a clear wide passage. At first, the line followed the roof as evidently Rob Parker had been looking for a way up to surface. However, the eight-metre tall passage was widest at the bottom and after 150 metres of swimming, the roof sloped down, forcing the line downwards into the wider section. At 40m depth, I encountered some loose line and took a few minutes to tidy this up. At 50m depth, the old line was secured to a rock feature on the floor and it ended. I attached a new line and pushed onwards into the unknown.

The tall rift I’d been following was gone and now the two-metre-high passage had a distinct cobble floor. Passing under an arch at 65m depth, I was confronted with a solid wall and a steeply ascending rift. I recognised this instantly from previous descriptions and the tell tail remnants of some blue line on the floor confirmed that this was the rift that Rob Parker had reported. This was the point which Gavin and Phil had dived to in 1996, when they confirmed that the rift did not lead on and instead the cave continued downwards.

Looking to my left I could see a tall rift which appeared to open out into larger passage. A suitable line belay was located in the rift and I then emerged out into a large, gently ascending passage. Rather than going deeper as expected, the cave began to trend upwards. The visibility was excellent in this new passage and I could easily see features over ten metres away. At around 40m depth, another solid wall blocked my way forward.

As I followed it upwards, I scanned the shaft with my handheld light and realised that the passage continued behind me. Soon I was swimming along a horizontal passage at 20m depth, which also ended abruptly with several large jammed rocks appearing to block the passage.

I was instantly disheartened but, as I got closer, I could see large dark enticing spaces between them. Ascending through the largest hole, I arrived in a spacious chamber. My NERD computer told me I was at 9m now and had some decompression to complete, so I located a suitable line belay and watched the timer count down.

After the stop was clear, I had two choices. Directly above me I could see the shimmer of an air surface, but to my left an enticing passage led off at 7m depth. I chose to follow the passage and soon found myself completing a final 6m decompression stop. The ten-minute stop passed quickly and while I waited, I swam around admiring the large cave passage, confident that soon I would find an air surface.

Pushing Further: The Discovery of New Sumps

Boosting gas
Boosting gas

Sure enough, when I picked up the reel, in less than 20 metres I rose up and surfaced in a good-sized lake. After securing the line and removing my cylinders, I clambered out of the water. A tiny flow of water filled the spaces between rocks under my feet, but otherwise the chamber was silent. About 40 metres upstream, another pool barred my progress. It was clear that this was another sump. I’d already swum about 400 metres and been underwater for 50 minutes, but the dive to this point hadn’t generated a lot of decompression, so I decided to carry on exploring.

One at a time I dragged my cylinders up to the new pool before re-kitting. Sump two turned out to be shallow and short. After no more than 40 metres of line had been laid, I surfaced once again. This time there was no moving water and absolute silence. I walked around the dry chamber, noting further potential leads for exploration, including two pools. The first, which I clambered down to, did not look promising and the second was at the bottom of a six-metredeep hole. Content with my success, I decided to turn back and complete a survey on the way out.

Returning to ‘Certain Death': Double Checking and Future Plans

Two days later, I was back in the ‘Certain Death’ sump for a second look. Firstly, I wanted to be certain I’d not missed anything underwater in the first sump, and secondly, I wanted to see if the pools were, in fact, sumps. One of these would be easily accessible, the other would need SRT kit and rope, which I carried strapped to my cylinders.

Swimming through the large clear sump in excellent visibility was fantastic and I was able to safely stray from the line is several places to peer into alcoves and look behind boulders. I’m now very confident there’s nothing I missed underwater.

On the far side of sump two, I was able to check out both deep pools and confirm my suspicions that one is, in fact, a sump and a possible underwater way on. On my own, however, it wasn’t feasible to lower all of my kit down the sixmetre drop into the water. After an hour and a half looking at every possible continuation, I had to conclude that future exploration here would require at least a team of two.

Looking Forward: Anticipating Future Expeditions

Once again, like in Cabeza Muxa, I’d found my way through the sump, but future exploration would mean more cave divers with more time and more equipment. With the potential for further cave exploration in the Picos de Europa, you can be sure that I’ll be back!

Underground Adventures In Spain 1

These articles were originally published in Scuba Diver ANZ #54 & Scuba Diver ANZ #55

Subscribe digitally and read more great stories like this from anywhere in the world in a mobile-friendly format. Link to article one & Link to article two.

Notify of

Inline Feedbacks
View all comments


Get a weekly roundup of all Scuba Diver news and articles
We don’t spam! Read our privacy policy for more info.
Scuba Diver Magazine
Scuba Diver Magazine
Scuba Diver Magazine is a global publication serving all the major English speaking markets in print and digital format.
Latest Stories
Would love your thoughts, please comment.x