Interesting stuff. I read a bit about BikePetes cave I think. Have they been using explosives to open the gaps up a bit?
I've not been far at all. Whitescar caves, Cheddar Gorge
.
Williams Pott in Yorkshire I think it was called also which was proper underground, muddy and wet but not deep. There's a secret cave I've been to that's opened by a lever which was great. I could find the valley I think but not the lever so that secret is safe and it's not very deep either.
All messing around like that be it underwater or underground takes skill and nerve, which I don't have much of.
I'd like to dive but my sinuses don't like it. The deepest is for a pair of sunglasses that fell off a boat, that was a bit far for no air but I got them.