My guess is the entire map is an interior, and the only actual terrain is the bumpy mess left down there.
......what
I think the floor there was not interior but a waterblock. A waterblock would be better to be used for the ocean floor for two reasons:
1. Better collision than interiors
2. It stretches on forever in the map
I drew that conclusion when I saw him bouncing up and down off the ocean floor. I haven't looked for myself though.
Is it possible to have two terrains in one map? Do not believe so.