Caves are often asociated with legends, stories, or local lure. Some of those stories are of historic origin, some are true, others are a sort of subterranean urban legends.
So here is a small list of common urban legends about caves and other underground structures:
One could ask how the 15km long tunnel was constructed during the Middle Ages, when 3.5m of tunnel required 1 year of work by a miner. To construct such a tunnel 40 miners must have worked 100 years. How should a passage have been formed and collapsed with no signs of a collapse. Most likely a look at a geologic map will tell you that natural formation is impossible.
The same with the treasure: could be possible, but if it really existed the people would have searched for it and found it long ago. It is more likely that there never was a treasure or the one who discovered it decided not to tell anybody. In most cases there never was a treasure, because as it goes treasures are rather rare at all, and a robbery is a good chance to cover an embezzlement by claiming the lost money was part of the booty.
These stories sound almost plausible at the first sight. But at the second sight, some math can prove them wrong. The probablility of such a story is extremely low, despite it sounding plausible. But the fact that stories with the same plot are told about hundreds of places all over the world makes them even more unlikely. It might be possible that something unlikely happened once, but that it happened this often is simply weird.
So how do you handle those stories? Dont be fooled and enjoy them as a nice piece of entertainment.