Go to Crystal Caves as Donkey Kong. Make sure you have some musical power. Use warp #2 and right next to the warp will be a building that can be open by playing Donkey Kong's drums. Open the building and enter. It should be a puzzle where you stomp on switches and rotate the room. Stomping on the switches in the correct way makes you go through the floor. This can take a really long time to do, so be patient- it does work. Here's how I did it: Go to the switch at the right. Stand at the edge of the switch closest to the wall and face away from the wall. Right after you stomp on the switch hold right (so you go in the direction of the wall which becomes the floor). I believe Donkey Kong has to go through the other switch to go through the floor, so this may take some tries... look at the screenshots for keys.
You will then fall through the floor and land on a gray rectangle. You will still be able to use your normal abilities, such as shooting, jumping, hitting, etc. You can use C up to examine the room above you. You can run on the black area surrounding the gray area, as well. However, if you run too far off into the black area, Donkey Kong will suddenly disappear and you'll be warped back to the beginning of the level.
Submitted by: kornyhatemachine