Atmospheric Franchthi Cave is one of Greece’s oldest and most significant Palaeolithic, Mesolithic and Neolithic sites, with occupation dating back over 40,000 years. The remarkable finds have revealed the transitions between all eras: the way hunter-gatherers lived from fishing, then organised themselves in groups, and finally transitioned to tool-using farmers. Many of the excavation's findings are in the archaeological museum in Nafplio. Offshore, you can snorkel among a submerged Neolithic settlement, Paralia; the shoreline was a further 7km away back then.
Signage in English is excellent and there's a small jetty. To get there, follow the signs to the cave for 4km (signs are 5km before Kranidi if coming from the north) to a beach. From here, you must walk along a trail over rocks for about 500m (these are marked by red painted arrows). At the time of research you could enter the cave freely. If this changes, access will be via the Municipality of Ermionida. You can get quite a good look through the security fencing.