A fully cleared out Ocean Monument that has been completely dried up.
Atlantis was simply a project done in spare time with the hopes of one day creating a Guardian farm at some point. However, most interest was lost, and it just became a giant empty space submerged under the ocean. This isn't to say that it isn't very impressive or interesting though. Having been created in the confines of an Ocean Temple, it has a few notable traits. For starters, no hostile mobs of any kind can spawn inside, even if it's dark inside. Second, it can theoretically be used to farm Guardians in the event that water blocks are placed inside.