Short answers, pulled from the story.
Plato introduced the story of Atlantis in his dialogues Timaeus and Critias around 360 BC. The narrative serves as a deliberate literary device rather than a historical record to contrast the ideal state described in his Republic.
The Atlanteans were the descendants of Poseidon and the mortal woman Cleito. They built a magnificent city with concentric rings of water and land fortified with walls of red, white, and black rock covered in precious metals like orichalcum.
The Atlanteans lost divine favor and sank into the Atlantic Ocean 9,000 years before Plato's time. This event served as a cautionary tale against hubris and the corruption of power within the narrative.
Aristotle, Plato's student, believed the island was invented to teach philosophy. Other ancient writers like Crantor argued the story was true while Proclus reported on Crantor's commentary though the original text is lost.
Ignatius L. Donnelly published Atlantis: The Antediluvian World in 1882. He claimed that all ancient civilizations were descended from a technologically advanced Atlantis that was destroyed by the Great Flood.
The scientific consensus remains that Atlantis is a fictional invention by Plato. Geologists and archaeologists have dismissed most theories noting that the Canary Islands and Azores have been uplifted over millions of years making them unlikely candidates for a sunken continent.