Largest underground waterfall: Gaping Gill
There are quite a few amazing underground waterfalls in the world. The largest of them all that is open to tourists to visit is La Grotte aux Fées in Switzerland. Shown in the image here, it is a gorgeous 253-foot tall waterfall that tumbles down into a cave that was once used as a sheep pen during Roman times but is now known as the cave of fairies.
However, there is an even taller underground waterfall in Gaping Gill. This is the second largest natural cave shaft in the United Kingdom, and the 360-foot high waterfall within the cave is the tallest unbroken waterfall in the U.K. This underground waterfall is taller than La Grotte aux Fées, however, you can't just waltz into the cave to visit. There are special tours held twice a year.
(And there may be taller underground waterfalls out there yet to be discovered!)