The first picture of Ocean Monuments, posted by Jeb on Instagram. Note that the room does not appear in-game.
The "Gold room" inside an Ocean Monument.
Sponge inside an Ocean Monument.
An Elder Guardian inside an Ocean Monument.
The entrance to an Ocean Monument.
Another Room in an Ocean Monument.
Map of a generated Ocean Monument.
Bizarre Ocean Monument on a customized world, with the sea level turned down.
An Ocean Monument.
2 ocean monuments next to each other.