For me it was undoubtedly in Pennygent last easter. I was in the derigging team but we caught up with the rigging team and a few of us shot ahead by simply slidding down a few of the rift pitches. After plenty of crawling about in water we found the next pitch down but we were never planning to bottom it so we didn't have enough rope.
So while a few of us waited for the others to catchup i saw a little crawl going off and thought it was worth a poke. After 10m of horizontal body sized, a 2ft diameter 4m pitch went down. I backed out of the crawl and attacked it feet first, enabling me to descend. At the bottom another 10m body sized led to another 2ft diamter pitch, this time about 5m. Again, feet first was the best option!
At the bottom of this was compact mud with a small dugout passage going 45 degrees down for about 5ft then i could see it going off horizontally. The ceiling was rock but the walls and floor were hard mud. I could here water off in the distance and a strong draught was coming through the hole.
I stopped for a minute to think...
The pitch earlier we didn't descend was about 11m with a stream at the bottom. I must be about that depth now!
Needless to say i did some quick removal of excess mud then took off my helmet and dived in. It was small, one arm infront holding my helmet, one by my side. After about 5m of wriggling i met sitting water making it much easier to slide along this tube. Once there was about 3 inches of air space left there was a t-junciton with what looked like a very low duck/sump each way, but there was a hell of a draught coming from one of them and a waterfall sounded very close.
I stopped for a minute to think...
Near the bottom of one of the deepest trips i had done to date, expecting to be out of the cave in about 3-4 hours time with derigging, this is not a very good place to try to kill myself, and i am VERY cold right now!
And so the decission was made to retreat as quickly as possible. Very easily acheived at first, just float backwards. However, as the passage really was body sized i pushed up a pool of water behind me through the solid mud tube. Needless to say, it turned into syrup and after 2 minutes i was half buried in slop. I carried on pushing backwards but it soon got much harder! I had to get up the 45 degree slope feet first. Given present circumstances, this was impossible. I used all the strength in every moveable part of my body but there was no way i could get more than my feet out of the top of the slope. I couldn't even hold my weight up the slope. And so i slid back down to take up shouting for a while.
After 5 mins still no result. Apart from i was getting even more tired from the uncontrollable shaking. My helmet was completely covered in mud so i was in the dark. And then i noticed a change. The water was flowing out of the mud and it was setting around me.
The shouting got louder...
After well over to long i heard crawling back up the last aven. My cries were answered. All i needed was a pull on my leg and i popped right out. Easy peasy. But i would have died with out it.
Lesson to be learnt ... dont do that again!!!