On the first glimpse, mainland Greece, but a close look reveals an extraordinary geography: the Peloponnese is a big peninsula, divided from Sterea Hellas by the long and narrow Gulf of Corinth. The only connection is the Isthmus of Corinth. For more than a century the Channel of Korinth allows the shortcut through the Gulf and Isthmus of Corinth (at least for small ships). So the Peloponnese became an island.
The Peloponnese is mainly made up of limestone. Numerous caves can be found all over, but the most famous karst area is south of Sparta on the Manos peninsula. Here are the oldest show caves of Greece and famous archaeological sites.
More karst can be found in the northern mountain ridge along the Gulf of Corinth, like one of the newest Greek show cave Ton Limnon in Achaia. But there are also karst lakes, karst springs and swallow holes.
More caves can be found at the eastern coast on the Argolis peninsula. Most famous is Franchti cave, with a complete series of archaeological remains from the last 30,000 years.