Go inside, to the basement (the place with green walls and random fireballs on them), get the the door where you can see water, and jump into it.
Swim all the way to the other side, and you'll see 2 pillars; Butt Stomp (A + Z) on each of them, and the water'll disappear.
Now get through that metal door, and go to the right, until you see a hole.