Many sources talk of the lost continent of Atlantis. Stories about Atlantis appear in the books of Plato, Pliny the Elder, Francis Bacon, Ignatius Donnely, and in the readings of Edgar Cayce, to name but a few. Even the ancient legends of the Mayas, the Incas and the Aztecs mention a lost continent, called Aztlan, lying in the area now covered by the Atlantic Ocean.
According to these stories, the Atlantean civilization was vast and highly advanced, capable of interdimensional communication. The Atlanteans apparently had the ability to control Sound and Light, and to transcend time and space. But through the misuse of these powers, the Atlanteans brought about their own destruction, and the continent sank.
Some authors do not only refer to Atlantis as a continent but also as an era in time, before the Great Flood (which took place between 11,000 - 10,500 B.C.), in which a combined extra-terrestrial / human culture flourished.

One of the most interesting sources of information on Atlantis, are the readings Edgar CAYCE gave to several people, and in which Atlantis and Lemuria are mentioned. Atlantis as the organized and highly evolved society would have flourished from 50,000 BC till approx. 11,000 BC, when its final destruction came with the Great Flood. (Read: E.E. Cayce, "Edgar Cayce on Atlantis.")

a) period before 50,000 BC

Some readings talk about how 100,000 to 98,000 years ago, mankind came into existence, as a physical race, with a distinction between males and females. Five races came into existence, about simultaneously, in five different areas in the World. Lemuria was one of them, as was Atlantis, the cradle of the 'red' race. (Interestingly enough, other readings mention that a first Terran race already walked the Earth about 10,000,000 years ago. That race, however, did not have a separation of the sexes, and was non-physical. They lived like 'projections' or 'thought-forms'. They completely disappeared without a trace or is it this race that evolved into mankind and took physical form about 100,000 years ago?).
The red race started to develop, spread, and grow, over a 50,000-year era of time into a highly evolved society, with contacts with extraterrestrial civilizations.
In that period of time, many strange things happened: experiments of non-physical life-forms taking animal bodies, or bodies that were half-human, half-animal... There were experiments with genetic manipulation and cloning, leading to the creation of a race of 'human machines.'
After a while, there was a rebellion by all these creatures, and a bloody war ensued. That is the time when the first emigrations out of the Atlantean Land occurred. People migrated to areas in the Pyrenees, Portugal and Spain, South and Central America and the Land of the Og(?), Egypt, ... The war then was fought between the 'Sons of Belial' and 'the sons of the Law of One,' using planes, laser technology, nuclear weapons etc... After a first war, came a second, a third and so on... till finally, about 50,000 BC, the wars were over, and Atlantis began a period of peace.

b) period from 50,000 - 10,000 BC

For 40,000 years, Atlantis was a highly organized and technologically evolved society, with contacts with extraterrestrial civilizations. Their technology and scientific advance were far beyond ours is today, and included e.g. crystal healing.
(If you consider that most current civilizations have a maximum age of some hundreds or maybe even some thousand years, whereas the Atlantean Civilization has existed for about 100,000 years, you can get an idea of how technologically evolved they were. Add to that, the contact with Extra-terrestrial civilizations that mastered space travel...)
Still the frictions amongst the people remained. The increase in knowledge was not accompanied by an increase in spirituality, or responsibility.
Eventually the tensions would lead to war again. In this war they abused the knowledge they had, which caused the Great Flood, that eventually destroyed Atlantis, around 11,500 BC.
Some Atlanteans felt the imminent destruction, and fled to Yucatan, Egypt, etc., establishing new cultures over there, in which they had to integrate with the local population. The ET-civilizations, too, knew what was about to happen. The Lyrans wanted mankind to be destroyed, so they could start anew with planet Earth. The Sirians however saw it differently and warned the other (=non-Atlantean) people on Earth. (Hence the stories about Noah). Those people built boats and survived the Great Flood. The ones living in the area of the Middle East, would establish the Kingdom of Sumeria.

Many predictions state that around this time, Atlantis should rise again. Cayce predicted that proof of Atlantis can be found in Egypt, Yucatan, the Pyrenees, and on several islands in the Atlantic Ocean. He also predicted that proof would be found first on the Island of Bimini. Even though denied by science, there are strong indications that confirm the existence of Atlantis.