The Forlorn Hollow

The Forlorn Hollow is the 2nd area in the game. It appears to be just some dark cave. Here you can find such enemies as Cave Nibblers and Stitches. The boss of this area is Paunchy Chops.

Also, there's a secret warp zone in The Hollow, on the level 2-1. Right after the start there is a platform that's being prevented from falling by a block. Normally you'd use this platform as a bridge, but to get in the warp zone, you have to make that platform fall on the left and then go under where it'd serve as a bridge. In the middle there's a breakable block.