Couple of incidents and places spring to mind in the Wheal Pendarves sett early 80's.
After a long hours slog through a very ochred adit with gonad cooling depth, we turned into a gently climbing crosscut and came to a 6x4ft winze/ladderway with a fully fitted set of of what looked like decent condition ladders and platforms as far as you could see down. I gingerly touched the top of the ladder and it squeezed through my fingers like putty.
We found a small pebble and dropped it down and all we could hear was plop, plop, plop as it went straight through each rotten platform. One of the lads had a waterproof casio with stopwatch, so we found a larger rock back down the tunnel and timed it. It took around 30 secs to hit solid bottom.
Second incident was complete 'pucker time' for one of the lads, though the funniest thing I've ever seen underground.
We came to a flooded shaft which at water level was opened out about 20 ft wide. One of the boys volunteered to take off his cap lamp and swim a rope across to the opposite level, so the rest of us could pull the gear over. He took off his helmet/lamp belt, tied a rope around his waist and let himself down into the water. He was about 6ft out when we heard a scream followed by a 2ftx2ft engine beam suddenly rearing 6ft out of the water right alongside him. Well he completed the swim and was out the other side in about 5 secs flat! At this point we were in hysterics having seen what had happened, had turned our lamps off and were rolling about laughing. The poor guy was sitting in the opposite adit in pitch black now, with no idea what had happened, aware of SOMETHING in the water and throwing swear words at all and sundry. Once we stopped laughing, we put the lights back on and after the disturbed mud had settled, peered into the water where could see some ironwork about 4ft below surface, which the beam must have been wedged under until he dislodged it with his foot. The beam was still bobbing up and down.
Eventually we all pulled across on the rope, but the opposite level was a dead end about 50 yards from the shaft.
I spent all my money on wine, woman and song. The rest I wasted.