This is an interesting webpage i've found during my internet travels last night.
It's a long read, and could take you anything up to an hour or so to read through it all.
Quite creepy in places, and i enjoyed it.
Be warned - not for the claustrophobic as it is a journal of a guy called Ted's caving experience and what he and his caving partner found, or didn't find.
I'll leave you to make up your own mind.
Here's two links, in case one of them is down.
Ted's Caving Page
Ted's Caving Page Backup