Go to Big Boo's Haunt. Now, find the red grated elevator that takes you down to the basement with a lot of water. Start up the elevator by stepping on it. Now, before you reach the bottom, jump off of the elevator and run under it. It will keep on going down and then smash Mario. However, Mario won't get hurt - he'll just sort of "pop" through the elevator. You can easily run out of it.
Once inside the elevator, you can get further things to happen. If you crawl, you will end up on top of the grating but make a weird splashing noise and see a splashing below the elevator. If you crawl into the wall, you will sometimes end up in a vertical position as if you were crawling upwards. Sometimes you will go into the wall and sometimes you will even fall apart; Mario's head will end up on his back.