The Raft is one of the four original supervillain prisons. Located deep in the ocean near New York City and known only by Nick Fury and a handful of high ranked S.H.I.E.L.D. agents, this prison contains the most dangerous supervillains ever captured regardless of being tech or radiation based or genetically altered. Various prison cells are desinged to each specific villain and shuts down their powers if they have any at all.
The only indication that The Raft is there is from a small garbage barge that acts as the only way in or out of the prison. The actual prison itself is deep below on the ocean floor.
The Raft was destroyed by Graviton in his anger on Fury as the other villains escaped the Raft.