On the first glimpse, this is 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 between the mainland and the huge peninsula is the Isthmus of Corinth, which is 5.5 km wide at its narrowest point. The peninsula is 220 km north-south and also 220 km east-west, looking like a hand with three fingers protruding to the south, and the thumb protruding east. For more than a century the Channel of Korinth allows the shortcut through the Gulf and Isthmus of Corinth (at least for small ships). When the canal was built in 1893 the Peloponnese actually became an island. It is quite inconvenient for modern traffic, there are only two road connections, across the isthmus and Rio–Antirrio bridge north of Patras. There are also numerous ferries connecting the mainland and the Peloponnese.
The Peloponnese is mainly made up of limestone, so actually the whole peninsula is karstified. There are karst springs, huge polje, karst lakes, and swallow holes. There are numerous submarine springs, as some cave systems run below sea level. Many of those are found along the eastern coast of Arcadia and in the Argolic Gulf.
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. In the northern mountain ridge along the Gulf of Corinth, is one of the newest Greek show caves, Ton Limnon in Achaia. The most famous archaeological cave is Franchti Cave, with a complete series of archaeological remains from the last 30,000 years on the Argolis peninsula.
Quite spectacular are the huge dolines at Didyma, two of them are signposted and easily accessible. Two more can be found on the hill in the north. With diameters around 200 m they almost qualify as tiankeng, but they are not deep enough. Nevertheless, they are breathtaking.
The archaeological highlights of the Peloponnese are the city Mycenae, north of Argos, towards Corinth, and the antique Olympia, the origin of the Olympic Games. Mycenae, Pylos and Tiryns are the three main cities of the Mycenaean civilization, all three are located on the Peloponnese. Then there is the Ancient Theatre of Epidaurus and the Temple of Asclepius who is better known as Aesculap, hero and god of medicine. His serpent-entwined staff has become a symbol for medical institutions.