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Otter Hole 20/6/26 (by Keith Biner) 13 Jul 2026 10:00 #22019
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8th of June, evening and a most welcome message appears on the Cambridge Climbing and Caving Club WhatsApp group - an invitation to take part in a trip to Otter Hole. I reply within minutes and wait for the upcoming deluge of requests to join from other CCCC members... After a while it becomes apparent everyone else is otherwise occupied or maybe put off by Otter Hole's somewhat grim reputation? We'll never know.
Lucasz fills me in on the rather sparse details. A car park somewhere at 07.45 the day before the solstice. Could be a lot worse from what I've heard.
A bit of research digs up some interesting facts, later confirmed by our very knowledgeable guide/warden Ade, who rather endearingly calls himself a 'Forest Caver' (as in (Royal) Forest of Dean Caving club).
Otter hole has, uniquely for the UK, developed entirely within dolomite ( CaMg(CO₃)₂ )as opposed to our better known limestone ( calcite or CaCO₃ ). That extra magnesium atom makes the crystal structure tightly bound and far less soluble in groundwater. So entire cave networks routinely take millions of years to mature. But again Otter hole is not even a typical dolomite cave. It formed much quicker due to tidal forcing and mixing, and its low altitude and rich soil above causes much stronger dissolving carbonic acid.
In the car park I was in the fine company of Lucasz, with whom I have spent numerous pleasant hours getting lost, Alex - who seemed keen (not many people visit Otter hole for a second time from what I could gather) and had the broadest accent I've ever heard - are you sure you're from Sheffield Alex? I would have put you 60 miles further North! Last but not least Ade, who was less gnarled than I imagined and not a rollie smoking masochist as I had somehow conjured into my head.
The walk down the steep heavily forested valley was a good warm up and a gated entrance at the foot of a cliff led to the first squalid muddy flat out crawl, nothing like getting stuck straight in!
After a bit of mud caked boulder hopping (lots of this further in) we reached the tidal sump, the cave's other claim to fame. We were doing an overtide trip where we were to pass the sump just before it flooded on the rising tide and then (hopefully) pass out as it recedes on the falling tide. This gives a longer trip than the between tides option and means you are committed to staying for a while! It also explains the necessity for a guide who is intimately acquainted with tide times and Otter Hole's many foibles. The streamway was a trickle so passing through the legendary point of no return felt slightly underwhelming. After a fixed ladder, a big step onto a slippery prominence, two boulder chokes with squeezes, a traverse, and an awkward tight chimney we eventually left the streamway. After one last long muddy crawl (Mendipian way), we reached the wash-off point — some brushes on a rock — you scrub the worst of the tidal mud off before entering the decorated sections. This is where the cave becomes a different place entirely.
Any descriptions of the formations fall far short of what lay ahead. In fact there was no real point in taking photos either as words/pictures just can't do justice to the sights beyond. Crystal pools, flowstone, curtains, gour pools — and then the passage enlarges into the Hall of the Thirty. Massive stalagmites — the largest in Britain — stalactites, columns, flowstone, orange and white, the floor a landscape of calcite formations. A taped path leads through it all. Usually you experience formations IN a cave. In large tracts of Otter Hole, the formations ARE the cave. The constant dialogue that accompanied our slack jawed wanderings says it all : "F*$%ing hell, this is F&*£%ing unbelievable!".
In any other cave one of these formations could easily have been the centrepiece of a trip but here they just kept on coming....
The highlight for me was crawling through a wall/ fused jumble of 'tites, 'mites, flowstone of the most pristine colours and cleanliness I've ever experienced.
We got to the end - a sandy convoluted low roofed shallow pool - and Ade took out his O₂ meter that he'd been checking since we started - 19.3% - down from atmospheric 20.9%. That might not sound like much, but oxygen transfer across the lung membrane depends on partial pressure, not just availability. A lower ambient concentration reduces the diffusion gradient, meaning your body has to work harder to maintain blood oxygen saturation. After nine hours underground with no quick exit, the trend matters more than the number.
The way back usually goes in a bit of a blur with a lot of caves but this one needed savouring.
Although the cave seems pristine, there are reminders of the surface - even 60 meters below Chepstow racecourse. One feature, horrifically beautiful, is Black stalactite - stained jet black from pollution, the result of a diesel tank leak from the surface. The fuel percolated down through the dolomite over time, staining the calcite formations and leaving the unmistakable smell of diesel.
Elsewhere a small adjoining water course was in the past drinkable, but housing developments have led to a new sewage treatment works which regularly discharge into the watercourse, now making the water an 'emergency only' option.
After a reasonably good route finding session on the way back a small crawl appeared in the wall in front of us, with sizeable flowstone formations hanging above our heads - strikingly organic in nature, translucent draperies of delicate folds and curtains of stunning colours. Ade said that was the way to crystal ball, with even more stunning formations. We were in two minds about whether to go or not but eventually decided to press on to the sump. I'm happy about the decision because although it would have been great to see this pristine part of the cave it now means I have a good excuse to come back!
Soon we were back to the streamway and a short detour upstream brought us to sump 2.
Sumps hold a fascination for me but I've only ever done Swildons 1 before so this was not to be missed.
Someone quipped "not really sure why you'd want to do that", well, I like being underwater and you don't get to be underwater in a cave that much, and also it's part of my belated campaign to become a "PROPER CAVER" before I hit decrepitude. Ade said it was about 12 feet or so to a small air chamber and recommended that I not go any further than that. My mind was doing its over-calculating thing - how will I know when I reach the alcove? I wouldn't be able to see anything, and I wouldn't be able to feel any air above with a wet gloved hand so what if I missed it? Ade assured me that my head would be clonking along the top of the roof anyway so it shouldn't be an issue. Alex advised that I not let go of the rope. Not bloody likely! I hung my bag on the end of the rope, took a couple of long, three-part breaths dimly remembered from a free diving course I did years ago, then waded in. In my haste I forgot to tell the others that I'd pull on the rope as some kind of signal - either good or bad once I got there.
In the end it didn't matter, after walking along the bottom and pulling the rope hand over hand above me I started walking up hill and my head popped out into a wide long chamber with the streamway disappearing into the distance, remarkably similar to the one I'd left. What a feeling! The urge to explore was dampened when I remembered that the others were expecting a quick in and out from me, so I had a look for a couple of minutes then plunged back the way I'd come.
I'm not sure if it was water flow but I got spun around and it turned out to be much easier to float and just haul my way back along with the rope above me, in no time popping out to find the others.
Divers have pushed through as far as sump 7, and in 2021 a white fish — likely a depigmented trout — was spotted in the pool at sump 5.
Following the streamway brought us back to the tidal sump.
The moon and sun had behaved themselves and the sump was open again on the way out, only a slight increase in water level to show that anything had changed since our entry.
Back into the boulder chokes and the going was laborious, the urge to see daylight not quite as compelling as the urge to see the cave's splendours on the way in. At one horrible twisty, tight, muddy, jumbled, heap of blocks Lucasz and I took a wrong turn, ending up slightly below the others under a boulder ruckle.
Lucasz managed to get his body through a gap above him that I could only just get my head through, and not having the energy to attempt a slippery climb I reluctantly backtracked to the less slippery climb I'd already attempted twice to eventually regain the others....
The last final flat out crawl to the surface was soooo much longer, slippier and narrower than on the way in. I am blaming the O₂ level which I'm sure had plummeted.....
We surfaced into lovely evening sunlight, and a cool down trek up the path through the overgrown forest to a bizarrely welcome wash off bathtub someone had piped high pressure stream water into, in the middle of a glade.
Definitely a trip that will last long in the memory. Otter hole makes you work hard but the rewards are worth it, simultaneously the most beautiful and the ugliest cave I've ever been in. Go!
Team : Ade, Lucasz, Alex, Keith
Total trip time : 9.5 hours
Lucasz fills me in on the rather sparse details. A car park somewhere at 07.45 the day before the solstice. Could be a lot worse from what I've heard.
A bit of research digs up some interesting facts, later confirmed by our very knowledgeable guide/warden Ade, who rather endearingly calls himself a 'Forest Caver' (as in (Royal) Forest of Dean Caving club).
Otter hole has, uniquely for the UK, developed entirely within dolomite ( CaMg(CO₃)₂ )as opposed to our better known limestone ( calcite or CaCO₃ ). That extra magnesium atom makes the crystal structure tightly bound and far less soluble in groundwater. So entire cave networks routinely take millions of years to mature. But again Otter hole is not even a typical dolomite cave. It formed much quicker due to tidal forcing and mixing, and its low altitude and rich soil above causes much stronger dissolving carbonic acid.
In the car park I was in the fine company of Lucasz, with whom I have spent numerous pleasant hours getting lost, Alex - who seemed keen (not many people visit Otter hole for a second time from what I could gather) and had the broadest accent I've ever heard - are you sure you're from Sheffield Alex? I would have put you 60 miles further North! Last but not least Ade, who was less gnarled than I imagined and not a rollie smoking masochist as I had somehow conjured into my head.
The walk down the steep heavily forested valley was a good warm up and a gated entrance at the foot of a cliff led to the first squalid muddy flat out crawl, nothing like getting stuck straight in!
After a bit of mud caked boulder hopping (lots of this further in) we reached the tidal sump, the cave's other claim to fame. We were doing an overtide trip where we were to pass the sump just before it flooded on the rising tide and then (hopefully) pass out as it recedes on the falling tide. This gives a longer trip than the between tides option and means you are committed to staying for a while! It also explains the necessity for a guide who is intimately acquainted with tide times and Otter Hole's many foibles. The streamway was a trickle so passing through the legendary point of no return felt slightly underwhelming. After a fixed ladder, a big step onto a slippery prominence, two boulder chokes with squeezes, a traverse, and an awkward tight chimney we eventually left the streamway. After one last long muddy crawl (Mendipian way), we reached the wash-off point — some brushes on a rock — you scrub the worst of the tidal mud off before entering the decorated sections. This is where the cave becomes a different place entirely.
Any descriptions of the formations fall far short of what lay ahead. In fact there was no real point in taking photos either as words/pictures just can't do justice to the sights beyond. Crystal pools, flowstone, curtains, gour pools — and then the passage enlarges into the Hall of the Thirty. Massive stalagmites — the largest in Britain — stalactites, columns, flowstone, orange and white, the floor a landscape of calcite formations. A taped path leads through it all. Usually you experience formations IN a cave. In large tracts of Otter Hole, the formations ARE the cave. The constant dialogue that accompanied our slack jawed wanderings says it all : "F*$%ing hell, this is F&*£%ing unbelievable!".
In any other cave one of these formations could easily have been the centrepiece of a trip but here they just kept on coming....
The highlight for me was crawling through a wall/ fused jumble of 'tites, 'mites, flowstone of the most pristine colours and cleanliness I've ever experienced.
We got to the end - a sandy convoluted low roofed shallow pool - and Ade took out his O₂ meter that he'd been checking since we started - 19.3% - down from atmospheric 20.9%. That might not sound like much, but oxygen transfer across the lung membrane depends on partial pressure, not just availability. A lower ambient concentration reduces the diffusion gradient, meaning your body has to work harder to maintain blood oxygen saturation. After nine hours underground with no quick exit, the trend matters more than the number.
The way back usually goes in a bit of a blur with a lot of caves but this one needed savouring.
Although the cave seems pristine, there are reminders of the surface - even 60 meters below Chepstow racecourse. One feature, horrifically beautiful, is Black stalactite - stained jet black from pollution, the result of a diesel tank leak from the surface. The fuel percolated down through the dolomite over time, staining the calcite formations and leaving the unmistakable smell of diesel.
Elsewhere a small adjoining water course was in the past drinkable, but housing developments have led to a new sewage treatment works which regularly discharge into the watercourse, now making the water an 'emergency only' option.
After a reasonably good route finding session on the way back a small crawl appeared in the wall in front of us, with sizeable flowstone formations hanging above our heads - strikingly organic in nature, translucent draperies of delicate folds and curtains of stunning colours. Ade said that was the way to crystal ball, with even more stunning formations. We were in two minds about whether to go or not but eventually decided to press on to the sump. I'm happy about the decision because although it would have been great to see this pristine part of the cave it now means I have a good excuse to come back!
Soon we were back to the streamway and a short detour upstream brought us to sump 2.
Sumps hold a fascination for me but I've only ever done Swildons 1 before so this was not to be missed.
Someone quipped "not really sure why you'd want to do that", well, I like being underwater and you don't get to be underwater in a cave that much, and also it's part of my belated campaign to become a "PROPER CAVER" before I hit decrepitude. Ade said it was about 12 feet or so to a small air chamber and recommended that I not go any further than that. My mind was doing its over-calculating thing - how will I know when I reach the alcove? I wouldn't be able to see anything, and I wouldn't be able to feel any air above with a wet gloved hand so what if I missed it? Ade assured me that my head would be clonking along the top of the roof anyway so it shouldn't be an issue. Alex advised that I not let go of the rope. Not bloody likely! I hung my bag on the end of the rope, took a couple of long, three-part breaths dimly remembered from a free diving course I did years ago, then waded in. In my haste I forgot to tell the others that I'd pull on the rope as some kind of signal - either good or bad once I got there.
In the end it didn't matter, after walking along the bottom and pulling the rope hand over hand above me I started walking up hill and my head popped out into a wide long chamber with the streamway disappearing into the distance, remarkably similar to the one I'd left. What a feeling! The urge to explore was dampened when I remembered that the others were expecting a quick in and out from me, so I had a look for a couple of minutes then plunged back the way I'd come.
I'm not sure if it was water flow but I got spun around and it turned out to be much easier to float and just haul my way back along with the rope above me, in no time popping out to find the others.
Divers have pushed through as far as sump 7, and in 2021 a white fish — likely a depigmented trout — was spotted in the pool at sump 5.
Following the streamway brought us back to the tidal sump.
The moon and sun had behaved themselves and the sump was open again on the way out, only a slight increase in water level to show that anything had changed since our entry.
Back into the boulder chokes and the going was laborious, the urge to see daylight not quite as compelling as the urge to see the cave's splendours on the way in. At one horrible twisty, tight, muddy, jumbled, heap of blocks Lucasz and I took a wrong turn, ending up slightly below the others under a boulder ruckle.
Lucasz managed to get his body through a gap above him that I could only just get my head through, and not having the energy to attempt a slippery climb I reluctantly backtracked to the less slippery climb I'd already attempted twice to eventually regain the others....
The last final flat out crawl to the surface was soooo much longer, slippier and narrower than on the way in. I am blaming the O₂ level which I'm sure had plummeted.....
We surfaced into lovely evening sunlight, and a cool down trek up the path through the overgrown forest to a bizarrely welcome wash off bathtub someone had piped high pressure stream water into, in the middle of a glade.
Definitely a trip that will last long in the memory. Otter hole makes you work hard but the rewards are worth it, simultaneously the most beautiful and the ugliest cave I've ever been in. Go!
Team : Ade, Lucasz, Alex, Keith
Total trip time : 9.5 hours
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