Hidden World

A Secret World is an area in a Metroid game that is created through a glitch. They are often one of two things. One thing they could be is a messed up version of a room, major area, or sometimes even the entire game. The other thing they could be is an area outside the boundaries of a room. These are often completely black except for the room you entered it from, which you can still see and reenter if you walk through the wall.

Metroid
In the original Metroid secret worlds were often found by using a glitch called the Door Jump or by using a cheating device. Some of the secret worlds were simply areas that got left out of the game. Others, reached by doing a "Flying" (using a cheat code or a hack) all the way up an elevator shaft are versions of the place you would be, with the graphic pallete of where you entered it from. The reason is that when you ride an elevator thats the games excuse to switch drives. So the secret worlds that you enter by flying up the shaft are really the actual place it resembles, the reason it looks that way is because the game didn't switch drives yet. This explains why most secret worlds in the game are stable.

Metroid II: Return of Samus
In Metroid II: Return of Samus Secret Worlds were often entered by using the Block Shifting Glitch. It sometimes will move blocks out of the way so you can enter the secret worlds. Like the original Metroid, they are versions of the room they are supposed to be, but filled with junk. Occasionally in these secret worlds you find disconnected parts of Save Stations. They will still cause you to save if you use either type of part. Also in the Secret Worlds you might find enemies that are made of the wrong sprites.

The Vertical Shaft Secret World is one example from Metroid II.

Metroid Prime Hunters
In the multiplayer modes of Metroid Prime: Hunters, you can exit the areas of a stage by using certain glitches or cheats. The simplest glitches are achieved by using an explosive weapon to blast you through certain spots in walls. The Combat Hall Secret World is reached in this manner.

Metroid Prime
In Metroid Prime, it is possible to get into a secret world in the Phendrana Drifts by jumping onto certain blocks protruding from the wall, as documented by.