The basic gist is: A research team finds some sort of artifact that could possibly prove that the Atlantis actually exists. They prepare and pack and start an exploration into the ocean to find further evidence. They ultimately do find the long lost city. However, they can no longer return to the surface and are stuck with the inhabitants of Atlantis. I suppose these inhabitants could vary from normal humans to mer-folk. Since they're already there, they decide to further investigate the reasons why the city disappeared into the ocean and how people could have survived. In doing so, they find out some dark secret. Anyone interested?