Go to Brinstar Depths as any character, but preferably one that can jump high or float. There are two times in which you can jump through the ground of Brinstar Depths and go inside of it. The screenshots help give a guide to where you can do this. The first area is where the rightmost pole/building-thing meets the ground, when it is stationery and at the bottom, to the right. The second place is where the leftmost pole/building-thing meets the ground, when it is moving from the left to the lower-left. Once you're inside, you can end up on another side of Brinstar Depths if you can get through the surface. However, if you run out of jumping power before you reach there, then you will fall down and die or get caught by a platform.
Submitted by: Zanto