A long limestone cave system on the central west coast, with a single entrance accessed by a small rope ladder and several internal chambers full of bats, stalactites, and the carved names of generations of Eleutherans (including, allegedly, the pirate Henry Morgan).
You can walk in and explore independently, but a guide is strongly recommended — passages branch quickly and lights fail. Bring a head torch, water, and shoes you don't mind soaking. There is no fee.