Atlantis isn't the only lost Greek city-state & Mount Olympus wasn't always a mountain.
Once upon a time the city-state of Olympias existed in the space where Mount Olympus now resides on Earth. Olympias was the last remaining city-state of a bygone era where Hellenism still ruled. When the Roman Empire, who had conquered the whole of the Greek peninsula some centuries earlier, was to be split in two the collective Olympian-Chthonic pantheon, with the help of an unknown outside entity, worked together to spare them the grisly coming fate. All the land that encompassed Olympias was carved from the very Earth on which it stood, a mountain erected in its place, and relocated across dimensions to eventually be placed in the Otherworld on a planet known as Terra. The inhabitants of Olympias, and others from the surrounding city-states who trusted in the advance warning given to the Oracles, were relocated alongside their land in 395AD. As far as any of those left behind can remember, Mount Olympus has always been a mountain.
Now a large island (or small continent, scholars are still debating), Olympias can be found on Terra off the southern coast of the continent of Europa. While rumors persist of the land belonging to either Elysium or Olympus, the inhabitants themselves claim no allegiance to either country and consider themselves an independent sovereign nation.