The caves are part of a large underground river system, some of which are partly filled with water forming a series of underground lakes. Evidence of human habitation in the caves have been found that date back to Neolithic and Paleolithic eras. To see these lakes you get into flat bottomed gondolas, with a gondolier negotiating the boat through the caves using a single oar. The caves are lit to highlight wonderful rock formations which are reflected in the still water creating an amazing visual experience. After the boat ride there is a walk back through some more caves, thankfully dry underfoot.
We went in early March some years ago when there were few visitors and we were lucky enough to take the boat ride in silence, an unforgettable experience. At peak times its likely you will need to wait to get in as the boats, of course, have a limited capacity.
Tip: The caves are on the west coast of the Mani, quite far down the peninsula past Areopoli. If you are driving from the Stoupa area allow more time than you might think when looking at a map. The road hugs the mountain side, zigzagging around headlands and into valleys, the journay took much longer than we expected.