There is another meaning, one which has long been familiar to me. Not so many miles away, in the Derbyshire Peak District, there are old mine workings as well as natural caverns, with some charming names. The "Speedwell Cavern" goes deep underground, and there it is possible to take a trip on a small boat. That has always sounded both mysterious and magical to me, so it adds an extra resonance.
I've been on the boat trip in Speedwell Cavern, many years back. Highly recommended if anyone is in the Peak District