Indonesia.
It was in Indonesia and the neighboring lands that Man, after emigrating from the semi-desertic savannas of Africa, first found the ideal climatic conditions for development, and it was there that he invented agriculture and civilization. All this took place during the Pleistocene, the last of the geological eras, which ended a scant 11,600 years ago. Though long by human standards, this is but a brief moment in geological terms.
The Pleistocene — a name which is Greek for "most recent" — is also called Anthropozoic Era or Quaternary Era or, yet, the Ice Age. During the Pleistocene and, more exactly, during the glacial episodes that happened at intervals of about 20 thousand years, sea level was about 100-150 meters (330-500 feet) below the present value. With this, a large coastal strip — the so-called Continental Platform (with a width of about 200km = 120 miles) — became exposed, forming land bridges that interconnected many islands and regions.
The most dramatic of such exposures took place in the region of Indonesia, precisely the spot where humanity first flourished. The vast expansion of the South China Sea then formed an immense continent, indeed "larger than Asia Minor and Libya put together". This is, as we shall see below, precisely what Plato affirms in his discourse on Atlantis, the Critias.
With the end of the Pleistocene Ice Age, the immense glaciers that covered the whole of the northern half of North America and Eurasia melted away. Their waters drained to the sea, whose level rose by the estimated amount of about 100-150 meters quoted above. With this rise, Atlantis sunk away and disappeared for good, along with most of its population, which we estimate, based on Plato’s data, at about 20 million people, huge for the epoch in question.