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Otter Hole - Beyond the Upstream Sumps


Otter Hole
Saturday 3rd July 2021

CDG divers were invited to support an ongoing filming project in Otter Hole, by capturing footage in the 165m of stream passage beyond Sump 4.

A recci trip was made on 12th June to ensure all sumps were open and lined before dragging in all the filming kit; the sumps were last dived in the mid-nineties, so we really didn't know what to expect. This was successful and a return was made on 3rd July to do the filming.

For those unfamiliar with Otter Hole, timings are critical. About an hour into the cave is a tidal sump (Sump 1), which can only be passed during a 6 hour window at low tide; the sump then closes for the next 6.5 hours over the high tide, so if you miss your window, you'll have a long wait! On this occasion, we were doing an 'over tides' trip, entering as the tide was rising and heading out again after the tide had dropped and the sump re-opened. This meant entering the cave 2 hours before the sump was due to close and exiting no sooner than 1 hour after the sump re-opened; so minimum 9.5 hours underground.

Despite leaving diving cylinders in the cave on the first trip, we still ended up with a decent sized bag each for the way in, making for steady progress through the entrance series which generally consists of a lot of crawling in squalid tidal mud and lethally slippery climbs between boulders. After just over an hour of this delightful fun, we reached the tidal sump, which was passed as a pleasant wallow under an arch and into the more pleasant passages beyond. Again, lots of climbing between boulders, but less crawling and the occasional respite of easy stream passage.

On reaching Sump 2, I made a quick free-dive with a length of old climbing rope to rig up a more appropriate free-dive line, allowing the non-divers to pass the short sump and assist with filming in the air-bell chamber beyond. By the time I returned, the 'dry' filming party had arrived to capture some footage of the divers kitting up.

Craig dived first to film the sump while visibility was decent, followed by Footleg making a free-dive again in hopefully decent(ish) viz, with the other divers brining up the rear. After capturing some footage between sumps, Craig set off into sump 3, again filming as he went, though visibility was poor even for the first though. By the time I dived (no. 3) viz was pretty much non-existent and the highly buoyant Peli-case (despite 2.5kg of lead in it) snagged on the roof at every opportunity.

The divers congregated in the air-bell before sump 4, where those who had been through before (myself and George) explained the underwater squeeze, that must be approached with the dive-line in the right hand. a sheer wall drops from the cave roof, as a step drops away in the floor, providing a narrow slot that is wide enough to pass in just one spot (and rather awkward nonetheless). Craig set off, line in left hand and after a short while returned, though his wellies had hardly left he air-bell throughout this exercise. We explained again that the line should be in the right hand, but this clearly didn't make it through the neoprene hood and cave noises as he disappeared again with line in left hand. On the third attempt, with line in right hand, Craig disappeared once more and did not return and we duly followed through to regroup in the streamway beyond; this was where our primary objective lay.


From here, Criag and I would leave diving kit and focus on filming, while Andrew and George would continue through to sump 5 with kit.


After a bit of faff with the equipment, we soon settled into a routine making steady progress up the streamway until a low section forces a move up into an oxbow above and through a bouldery squeeze before descending back to the stream. This would be no great challenge if it were not for the dive cylinders, providing some good entertainment for those of us without. Immediately beyond this squeeze, a climb leads to a possible connection to the cave above (accessible to non-divers); a secondary objective was to test an audio connection with a team above, however with the noise water, it was not possible to detect any sound of cavers above. I did however find a length of bang wire half way up the climb, which had fallen through from previous digging activities above, so it's clearly not far to go to the breakthrough.

Continuing upstream, we reached sump 5 where George and Andrew would dive while Craig and I took some still photographs in the streamway. While setting up for a photo at the dive base, I took one of the lights over to the side of the passage, intending to put it just below water and light up the sump pool and to my amazement spotted a pure white fish, perhaps 8" long resting in the calm water away from the main flow. The fish clearly wasn't so pleased to see us as we were to see it, but we did manage to get some close-up video before it swam off (it was spotted back in the same spot a little later on, so can't have been too upset).

George dived Sump 5, and entered Sump 6; having now lost the flow, the viz quickly disappeared and with several loose lines floating about and one entanglement already suffered, he decided to return.



Andrew dived next with the camera and lights to capture some footage between the sumps. The divers then overtook us, as Craig and I carried on with the photography, regrouping a short while later at Sump 4 dive base. Below is a selection of photos from along the way:










With the mission accomplished, we now just had to get ourselves and all the kit back out of the cave... To keep the Peli-case under control in the sumps and prevent snagging, I opted to carry it in my hand rather than dragging it on a lanyard as I had before. In zero viz, with a big floaty box in one hand and taking care not to lose the line with the other, finding the wide point in the Sump 4 slot was a tad awkward; fortunately the sump is short and the line is fairly tight, so you're never too far away from where you need to be and after shoving the case ahead of me, I wormed my way through and back to the air-bell. From here, Sump 3 was uneventful, with the case firmly in-hand and likewise for Sump 2.

Back on the dry side of sump 2 at about 5pm, the filming team conducted some brief interviews with the divers, while we scoffed a bit of chocolate and some pork scratchings (our first nourishment since breakfast), and packed up kit. We opted to leave two sets of cylinders in the cave for the next dive, which free of filming duties, should see slightly quicker progress. Even so, we still ended up with one more bag than pairs of hands to carry, however one of the other group (who had been doing some photography and interviews elsewhere in the cave) returned with spare batteries for one of our group and was able to relieve me of my second bag after less than 10 minutes.

Steady progress back through the cave saw us meet the other group shortly before the tidal sump, which was now nicely open. They had been waiting nearby in a more comfortable spot, rather than at the sump pool where it is a horrific mess of tidal mud!

Variously regrouping and splitting as obstacles dictated, the extended party (of at least 12, but possibly more) wormed their way back through the muddy crawls and squeezes, arriving back at the entrance some 10.5 hours after we'd first entered; more like 12 hours for some, who'd headed in earlier than the main group.

This trip not only marked the completion of a 7 year filming project in the cave, it also marked the 250th trip for Paul Taylor which, considering the grim entrance series, is about 249 more than most people would ever wish for!

Champaign was served to 'everybody brown' at the entrance, before the long slog back up the hill to the car park.

A great trip and really looking forward to Paul's film when he's finished the post-production in the next seven years perhaps (y)

Divers: Peter Hall, Craig Holdstock, George Linnane, Andrew Rice.
Sherpas: Monica Bolani, Tom Chatterton, Becca Kirkpatrick, Maria Lynch.
Filming team: Paul Taylor, Footleg and many more who were not directly involved with our part of the project.
Photo credit: Craig Holdstock


Craig has now sent me a still from the video footage. This was spotted in the pool at the downstream end of Sump 5.


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Well-known member
PeteHall said:
Craig has now sent me a still from the video footage. This was spotted in the pool at the downstream end of Sump 5.

Looks like salmon or sea trout, but hard to tell from the photo - they look pretty similar.


I'd assumed that it's a trout, but without eating him, I'm not too sure. I expect I'd get in trouble for that though  :LOL:


Well-known member
The clue (apart from the spots) is the tiny fleshy adipose fin just in front of the tail - members of the trout/salmon families all have it. And grayling, which is a peculiar 'inbetween' freshwater species. It doesn't have rays like regular fins, and is possibly more of a sensory organ than a propellant, though it may provide additional stability in fast currents:


Paul Marvin

As an ex Angler I would be pretty certain thats a juvenile trout bit short on colour because of the environment

One place I would love to visit !


Otter Hole - 25th September 2021

Following our previous two trips, I met up with David Hardwick,  who has published a paper on the hydrology of Otter Hole. Dave quizzed me on what we'd seen upstream and presented me with various theories about where the water comes from, with intriguing possibilities for further exploration.

Interestingly the cave is fed by at least 4 different streams and it is not yet known where in the system they all join. Further to this, Peter Bolt had perceived a lower flow in sump 6 than coming out of sump 5; this suggested an inlet part way through sump 5.

Studying the survey,  I reasoned that any inlet was most likely to be on the right hand side and it was proposed for two divers to return, with one scouring the right hand wall of sump 5, while the 2nd made an attempt on sump 6, 7 and onwards.

Meanwhile, two others would go to the far end if the fossil cave, up a passage known as Tunnels Left, where a series of sumps was last attempted in the 80's. The purpose of this trip would be to place a dye detector and assess the route for carrying diving and camping equipment for a push next year.

With four cylinders in the cave already, we'd be bringing in three more bags of diving kit, plus food and sustenance for a long day underground. On the way out, there would be less food to carry, but four extra bags of diving kit, so a large team of porters would be required. Jann Padley of GSS did a sterling job pulling together a team if volunteers to help and with a week to go, all looked to be set...

But as with all the best laid plans,  that wasn't to be. One of the divers pulled out to help with the Three Counties trip, one of the Tunnels Left team got stranded overseas with work, one of the porters injured themselves and another also had to pull out.

After a rethink, I managed to rope in Dave Hardwick to join the Tunnels Left trip (to be fair, he offered to go) in my place. Craig Holdstock would carry on as planned on this aspect of the trip.

Jann found a replacement porter and after a call for help around my club Rob Thomas also offered to help porter; we were back on.

Until the day before when the second diver phoned me after a skydiving accident, now unable to cave...

Another rethink was in order.

Since the streamway between Sump 4 and Sump 5 is quite mobile underfoot on various boulder slopes, I was keen to have two divers beyond Sump 4 in case of an accident. Fortunately, Rob, who was staying at the Wessex, was able to cobble together enough kit to dive, but we were now down a porter.

In the end, we settled on taking in a 2 litre cylinder to supplement the kit in the cave. This would be enough to get Rob through Sump 4 and assist with my kit in the streamway up to Sump 5. I would carry on as planned, scouring the right hand wall of Sump 5 for an inlet, and place a dye detector in the start of Sump 6, but we'd save a push dive for another day. This would mean that most of the cylinders would still be full and could stay in the cave for next time, as we were short on porters.

Tide times meant an early start and seven of us met in the car park at 05.30 on Saturday 25th. After a bit of faffing with gear, we were packed and on our way down the hill and into the cave at 06.30, bang on schedule.

Craig and Dave set off first as they had the furthest to go, with the rest of us following, with a bag of kit each. By 08.00 we had arrived at Sump 2, where Rob and I kitted up and were off by 08.30, with an estimated two hours before we'd be back.

Since Rob had not been before, I let him dive first to get some visibility in the low wide sumps; I followed, with about six inches of visibility...

Dumping Rob's kit, we carried a cylinder each up to Sump 5, where I kitted up again, for a slow dive through, to look for any possible inlet. I was expecting another low wide silty sump, with poor visibility. I was expecting to be groping about trying to feel for any widening as the bedding pinched off to the side, so imagine my surprise, to find myself standing upright (I was overweighted and not wearing fins) in a huge passage full of crystal clear water!

It must have been 6m high in places and over 3m wide. The whole place completely illuminated, I could see every detail!

If only I'd known it was going to be like this, I'd have brought the fins we had stashed at Sump 2, I'd even have ditched some lead and taken a more neutrally buoyant approach. In a passage that size,  even I could have avoided stirring up the silt!

Anyway, that was not to be, I stirred up great clouds of muck as I walked along, before climbing on ledges to follow the line to surface beyond the sump.

Regardless of my silt-stirring, I got a really good look at the whole passage and was able to confirm that there is categorically no inlet along the length of the sump, and I didn't have to resort to a fingertip search!

Beyond the sump, instead of the streamway I'd expected,  I found myself in a large air bell, with no dry land and 6m of water under me and I was still well overweighted...
I managed to remove my weight belt and popped it on a narrow ledge; now able to float, I was able to inspect my surroundings a little better.

To the left, a virgin passage headed off above water. This had been noted by previous divers, but never entered. To get into it would involve removing diving kit and digging over a mud bank and right now, this didn't appeal. It would have been criminal to dump all that silt into such clear water!

Opposite this, to the right, the dive line was tied off to a flake of rock and disappeared off diagonally down a narrow rift into Sump 6, however, by descending straight down at this point, a wide tunnel could be seen heading off, with the line pulled into the rift above. With such clear visibility, it was easy to see what was going on, but I can imagine in more typical conditions (our trip came after a very dry period with unprecedented low water levels) this area could be pretty confusing. I made a mental note to fix the the dive line back to the left hand wall, half way down, to give a better route into the passage below,  before attempting this next sump.

Back in the air bell, I shot a small bit of video, before finding the pre-tagged dye detector destined for Sump 6 and fixed it securely to the line.


After this, I put my weight belt back on and returned to Rob, with reasonable visibility, as the water had cleared a bit while I was faffing in the air bell.

On my return, Rob borrowed my kit and popped through to the air-bell for a look, while I fixed the next dye detector downstream of Sump 5. On his return, Rob and I split the kit to carry and set off out.

While I was gone, Rob had taken a short wonder downstream and had noted a couple of interesting holes that might be worth a look some time.

Back at Sump 4, we both put our own kit on again and dived back out, reaching the dive base at Sump 2, just 5 minutes after the porters had returned.

Rob hadn't used much air off the 2 litre cylinder and hadn't used the 3 he was carrying at all, so both these could stay in the cave.

I'd also not touched one of my cylinders, but the other was now down to about 100 bar, having been used on two previous trips and by both of us for Sump 5, so the full one could stay in and the other come out.

After repacking kit, we had a bite to eat and a cup of coffee before setting off out, with more or less, the same amount of kit we'd brought in, fixing the final dye detector a short distance downstream.

Wandering along the passage,  I stepped off a rock into what had been ankle deep water and disappeared up to my chest. We'd reached the tidal zone and for the first time, I was seeing it at high tide!

As we were wearing wetsuits Rob and I decided to swim on, taking two bags each. The logic being, we'd plod slowly on and the others would catch up quicker without bags and we'd all get out sooner.

At about 1pm, we reached the tidal sump.
At about 1.45 the others arrived.
At about 2pm (bang on schedule) the tidal sump opened.

Again, Rob and I opted to push on ahead with the bags, while the others waited for the tide to drop, so they'd stay dry.


Rob and I eventually emerged at 3pm, with Jann, Ross and Paddy surfacing half an hour later, for the long slog back up the hill.

By 5pm, we were starting to wonder where the Tunnels Left team had got to and agreed we'd need dinner before going back to look for them, but eventually we decided to walk back down the hill and were very happy to find them at the kit washing point only 15 minutes from the car park.

They too had had a successful trip, reaching the end of Tunnels Left and finding water lower than it had been seen before were able to duck through into a considerable distance of passage that is usually blocked by a sump, before eventually arriving at the beautiful blue pool marking the end of their trip and our objective for next year.


Well-known member
Great write-up, really interested to see how this project develops. Thanks.

And seeing your logistical challenges to get the team there really underlines again the impressive achievement of the 3 Countries Traverse team this weekend  :eek:  (y)


Rob said:
And seeing your logistical challenges to get the team there really underlines again the impressive achievement of the 3 Countries Traverse team this weekend  :eek:  (y)

Yes indeed! Getting that many people in the right place, with the right kit, at the right time is in my opinion a far grater feat than actually doing the caving! And that's not to do down the challenges of the caving!

Part of Saturday's trip was a recci for next year's (or some unspecified future year's) push in Tunnels Left. With a bit more knowledge of the route and the conditions, we can start to plan the logistics, which will be a little more challenging than what we've done so far!


New member
Great trip report. Sounds like an interesting project and look forward to hearing how it progresses.

Sent from my SM-T290 using Tapatalk



Active member
Excellent report.

I found it hard enough getting myself through the entrance series; I can't imagine what it would have been like dragging diving kit.  Sterling effort.


Otter Hole - 26th March 2022

Nearly 6 months to the day since our last trip, the weather was looking good to get back in and collect the dye detectors we'd placed. Ideally these would have come out sooner, but some inconsiderate b@stard decided to fall down OFD taking out an opportune period of time for some of us and the next season for himself! Despite this, George did kindly fill up a cylinder for me the night before, which is now stashed up at Sump 2 for a push later in the summer.

This trip was mostly not 'beyond the upstream sumps' as the thread title, but as it's part of the same project, I'm posting it here for continuity.

I met with Andy Thompson and Daniel Duggan at 7am and we were kitted up, down the hill and into the cave by 08:30. 45 minutes later we were at the tidal sump, where only the eyehole was open (see previous report a few posts back for a video of this entertaining obstacle) and within 2 hours, we were up to Sump 2. Here we washed off and left the two bags of diving kit we'd been carrying, leaving us with one bag containing lunch, a camera and a small tripod.

From here was new territory for all of us as we left the streamway and headed up into the fossil cave. This was a rare privilege, as Otter operates a leader system, so few get to "discover" the wonders that lie beyond for themselves; we had received special dispensation (and a key) to access the cave as part of the ongoing diving and water tracing project.

Following the approximate directions received the night before, we worked our way through the squeezes, bouldery wriggles and sandy crawls to emerge in the main fossil cave,. Rounding a corner, we were met with an enormous stal boss, then another, then another. As the cave continued we could hardly believe it possible, but at each corner the formations became more and more incredible until we arrived in the 'Hall of 30', undoubtedly the most beautiful chamber in the country, and probably one of the best in the world.

Despite my instructions at the start of the trip that we were to focus on the main objective and not waste time hanging about looking at pretties, we were awestruck and spent a considerable time taking the place in, before pressing on into the cave.

From here, it was like walking through a dream, from one incredible place to the next, on and on with no let up to the beauty of the place and seemingly no end to the cave.

Eventually, after 5 hours underground, we arrived at 'End Chamber' where a climb down over boulders brought us to a dingy streamway, worlds apart from everything we'd seen for the last few hours. After a little faff, we managed to locate the dye detector, grabbed a quick photo and turned back.

Since we were now in no rush for the tide, we allowed ourselves a bit of time to take a few photos as we headed back out. These do the place no justice at all, but they do at least give an impression of the place.

9.5 hours into the trip, we were back at Sump 2, where I kitted up to collect the dye detector from Sump 5, leaving Dan and Andy with a flask of coffee and a promise to not be too long. Fortunately, I'd placed this dye detector, so there was no faff finding it and I think I was back within about half an hour.

Shortly after 8pm, we were back at the tidal sump, again needing to pass through the eyehole as the main passage was underwater, and by 9pm we were back outside, mission accomplished.

The team at the end of the cave:

In no particular order, some of the pretty bits: