Story
"Here be monsters": this expression has always been used to speak of the ocean - a dark, impenetrable world that can only conceal unimaginable horrors that the human can't see...often not until it is too late. A series of unexplained accidents happen at a submarine platform 6,000 meters beneath the surface and, in response, a group of scientists and cadres are sent to investigate. Before long, the members of the party start to get haunted by horrifying, elusive visions...and the rash of brutal deaths once again resumes.
Synopsis via AniList (English).