These extraordinary flooded caves, inhabited since Neolithic times, lie 11km south of Areopoli and are signposted near the village of Pyrgos Dirou. The entrance is on the beach. Guides basically only speak Greek, so if your group doesn't you'll be treated to a half-hour's silent, eerie glide by boat through the cave's many passages, giving you time to admire the stalagmites and stalactites, many of the latter as fine as gossamer threads. You tackle the remaining 300m on foot.
Abandoned as human habitation in 4 BC after an earthquake, the caves weren't rediscovered until around 1895. Then in 1949 the local husband-and-wife speleology team of Anna and Yiannis Petrocheilou began to systematically explore the caves, now estimated to be around 14km long. Underwater exploration continues to this day.