The Twilight zone lies just below the diving range of recreational scuba diving yet above the range of where submersibles are used.
The underwater Twilight zone is one of the least explored regions of our planet. Diving the ocean's twilight zones is beyond the range of most scuba divers. It begins at about a 200ft – 500ft range below the surface and requires special training and diving equipment such as mixed-gas rebreathers.
Skeletal coral structures remaining from the ice-age provide an abundance of habitats for dozens of species at these levels. Just outside of the range of easy exploration new species are constantly being discovered.
Pressure in the Twilight Zone – California Academy of Sciences