Atlantis is a legendary kingdom originating from a story by the Greek philosopher Plato, in which a vast and powerful maritime civilization rises in the Atlantic Ocean, but in time it's leadership becomes expansionist and hubristic, earn the ire of the gods and as such decide to sink it. The story could be thought of as the classical age version of the War of the Worlds, a tale of a vast superior external civilization invading and enslaving those less advanced to it with a political allegory in in mind. During the 20th century, many people believed Atlantis was a real place, and mounted searches to find it, but belief in its reality has waned over the decades.
It is most commonly referred to on /tg/ for its popularity in Sword & Sorcery novels - thanks to its strong historical presence in the Hyperborea setting - and for its frequent appearances in comic books; both Marvel and DC have used the idea of Atlantis having survived by its people adapting into aquatic humanoids to generate superheroes, which means Atlantis commonly appears in superhero RPGs like Mutants & Masterminds.
The idea of Atlanteans existing as amphibious humanoids has even made its way into Dungeons & Dragons and Pathfinder; the Aventi race (and its Gillmen counterparts), and the Triton race in 5th edition, both are from the D&D equivalent of Atlantis.
In the works of JRR Tolkien, a prominent civilization is Numenor, an advanced civilization of humanity which has a golden age of prosperity but eventually turns to violent expansionism which . In Quenya the word for the downfall of Numenor is Atalantie.