Here's my favourite Oh shi* moment (well, my favourite caving one . . . I think the nearly-falling-out-of-the-back-of-a-high-speed-train one beats it, but it's got nothing to do with caving):
Many years ago I was exploring a resurgence cave in Northern Spain, from which came a substantial stream of very cold water. After about 200 metres there was a sump, but in one wall of the cave just before the sump there was a roughly circular hole maybe 1.2 m in diameter that was emitting a strong, cold draught, which led us to believe that it should be the start of a sump bypass. This proved to be the case, but it involved a lot of grovelling. Anyway, eventually we reached the continuing stream, which at this point was a canal that led to a lake, on the far side of which was a superb vadose canyon heading into the heart of the mountain. At his juncture we decided to call it a day.
The next day we returned with a bigger party to continue the exploration; I’d got it into my head that the first sump was probably quite short, so I took along a diving mask to see if I could dive it. Sure enough, when I stuck my head under the water (did I mention that it was very cold?) I could see an air surface a short distance ahead, so I ducked under and found myself at the start of the aforementioned canal. So there you had it – a choice between a ~1‑metre dive and about 100 m of grovelling, which constituted a sump bypass. In point of fact, since you were up to your chest in water before you’d even left daylight, and you had to swim along the canal and across the lake anyway, there wasn’t much point in doing the bypass.
So we followed this magnificent vadose canyon upstream until we reached the inevitable sump . . . at which point, I decided in a moment of madness that having passed one sump, I could pass another. This one, however, was quite different; the passage here was about 3 m wide, the roof shelving down into the (very cold) water. Still, in I went, and again I could see the glint of a water surface some distance ahead, so I swam into what turned out to be an air-bell after maybe 2½m. At this point I decided this was getting a bit silly, and decided to return . . . but now we reach the ‘Oh shit’ moment – I couldn’t get back. What had happened was that I’d followed the sloping roof of the cave down into the sump, but when trying to dive back down to get beyond the lip (see diagram) I couldn’t overcome the buoyancy of my wet-suit combined with lungs full of air to get down deep enough to pass the lip. So there I was, treading this (vey cold) water, thinking, ‘What now’. Bizarrely, I remember that I tried to compute the volume of this air-bell and find out (based upon some notion of breathing 1 cubic foot of air per minute) how long I had to live; the answer? – not long.
I thought of various strategies, including stripping off my wet-suit and leaving it there to return later (or send someone else back) with a rope to drag it out. Eventually I hit on the idea of breathing out before diving, in order to reduce my buoyancy. So ‘taking a deep breath’ – or, rather, exactly the opposite – I tried again, and this time succeeded in getting below the lip, to arrive back in the ‘final chamber’ gasping for breath, freezing cold and shitting myself.
