Arcadia is a central region of the Peloponnese in Greece. It is one of a number of locations that have been proposed as inspiring elements of Plato’s Atlantis story. One proponent is Oliver D. Smith who argues that “Plato based his king Atlas on a mythical Arcadian king of the same name and his main inspiration for Atlantis was the town Methydrium and nearby city Megalopolis, both in Arcadia.”
This, together with some other details, such as similarities between the ten kings of Atlantis and the ten kings of Arcadia led Smith to his conclusion(a).
Capes Matapan (Tainaron) and Maleas in the Peloponnese are the two most southerly points of mainland Greece. They have been proposed by Galanopoulos & Bacon  as the Pillars of Heracles, when the early Greeks were initially confined to the Aegean Sea and the two promontories were the western limits of their maritime knowledge.>They note that
“This has been the subject of some interesting conjectures. Nearly all the labours of Hercules were performed in the Peloponnese. The last and hardest of those which Eurytheus imposed on the hero was to descend to Hades and bring back its three-headed dog guardian, Cerberus. According to the most general version Hercules entered Hades through the abyss at Cape Taenarun (the modern Cape Matapan), the Western cape of the Gulf of Laconia. The Eastern cape of this gulf is Cape Maleas, a dangerous promontory, notorious for its rough seas.
Pausanias records that on either side of this windswept promontory were temples, that on the west dedicated to Poseidon, that on the east to Apollo. It is perhaps therefore not extravagant to suggest that the Pillars of Hercules referred to are the promontories of Taenarum and Maleas; and it is perhaps significant that the twin brother of Atlas was allotted the extremity of Atlantis closest to the Pillars of Hercules. The relevant passage in the Critias (114A-B) states:
‘And the name of his younger twin-brother, who had for his portion the extremity of the island near the pillars of Hercules up to the part of the country now called Gadeira after the name of that region, was Eumelus in Greek, but in the native tongue Gadeirus — which fact may have given its title to the country.’
Since the region had been named after the second son of Poseidon, whose Greek name was Eumelus, its Greek title must likewise have been Eumelus, a name which brings to mind the most westerly of the Cyclades, Melos, which is in fact not far from the notorious Cape Maleas. The name Eumelus was in use in the Cyclades; and the ancient inscription (‘Eumelus an excellent danger’) was found on a rock on the island of Thera.
In general, it can be argued from a number of points in Plato’s narrative that placing ‘the Pillars of Hercules’ at the south of the Peloponnese makes sense, while identifying them with the Straits of Gilbraltar does not[p.97].”<
Luciano Chiereghin is an Italian researcher who has a great interest in the history of the Po Valley, both ancient and modern. In his 2007 book Atlantide al Microscopio (Atlantis Under the Microscope) he has the plain of the Valley as the location of Atlantis (=Hyperborea) and specifically the ancient town of Adria. He also proposes that Majorca, Sardinia, Corsica, Sicily, Crete and the Peloponnese constituted the island territories of Atlantis.
However, he is not the only one to link this region with Atlantis, as Morven Robertson published a book in 2015 with a similar theme. Both authors were drawn to the Po Valley by its size and its proximity to the magnificent mountains of the Alps, which protect the plain from the northern winds.
Diego Marin has favourably reviewed Chiereghin’s book(a).
Pavlopetri is the name given to a sunken Greek city off the southern Peloponnese,>discovered in 1904 by geologist Phocian Negris and rediscovered as recently as 1967 by Nicholas Flemming.<An Anglo-Greek team of archaeologists have dated the remains to between 2800 and 1200 BC and as such are referring to it as the oldest (known) submerged city in the world(a)(b). This dating places it before the time of Plato and so it did not take long for commentators to suggest that it was possibly the inspiration behind aspects of Plato’s Atlantis narrative. However, the number of known submerged cities in the Mediterranean has been numbered at around 200. Every time one is discovered there is usually an attempt made to associate it with Atlantis, which fades when it is realised that it fails to match many of the other descriptive identifiers noted by Plato.
What I find interesting about Pavlopetri is that apparently it is never referred to in any classical Greek literature. Sceptics often claim that the reality of Plato’s Atlantis is undermined by the fact that Plato is the only ancient author to mention it and yet, Pavlopetri, unknown until the last century, does exist without any known written reference to it!
An October 2011 BBC documentary City Beneath the Waves Pavlopetri revealed that the port city was more extensive than originally thought and that it traded with other Aegean states particularly the Minoans on Crete.