You found it, Sly! This is where Raleigh's gang of crabs stashes his sunken treasure.
― Bentley to Sly a he enters the crab's lair[src]
Sly approached a submarine and dived down into an underwater room, where Raleigh's gang of crabs would stash his sunken treasure. Sly had to use the submarine to collect forty treasure chests in order to gain the treasure key. Sly had to do whatever it took to prevent a crab from taking a treasure into a tube, or the mission would be a failure.