A secret world (SW) is any location in a Metroid Prime game where it's possible to get out of bounds. Some of the more well-known secret worlds which are useful in Metroid Prime speedruns are the one in Gathering Hall, which is used in the Ice Beam Before Flaahgra (IBBF) wallcrawl, and the one in Phendrana Shorelines, which is used in the Wave/Sun wallcrawl.
For a complete list of secret worlds, click the respective links below:
Types of Secret Worlds
How you get out of bounds in any given room is a matter of what options the room gives you. As such, each room that has a SW may have a different way of getting out of bounds. This section will detail some of the most common ways that secret worlds are utilized in the Prime games.
In some rooms, the developers didn't think there would be any way to get to the ceiling, so they sometimes would leave the ceiling open. In Prime 1, for example, the room Gathering Hall has an open ceiling which is frequently used to get out of bounds in multiple speedrun categories.
Hole in Collision
Even in rooms without an open ceiling, there may still be holes in the collision which can be passed through. The best example of this is in Echoes, in the room Hive Chamber A, which is the basis for Item Loss Skip.
Reposition by Morphing
With Screw Attack
In the original release of Metroid Prime 2: Echoes, the collision model used for Screw Attack is the same as the Morph Ball. Due to the way SA works, this means that there are some areas where Samus can be forced to "unmorph" into areas that would otherwise not be possible, opening up many secret world possibilities. Most Morph Ball tunnels are vulnerable to this, such as this one in Dynamo Works and Hive Dynamo Works.
It can also be used in certain small spaces to get repositioned out of bounds, such as this spot in Hive Energy Controller.
The term "secret world" was originally coined by players of Metroid II when they found ways to get out of bounds, leaving rooms jumbled and leading them to believe they had discovered "secret worlds."
Secret worlds are currently allowed in every category unless explicitly banned. This was not always the case, however, as secret worlds were banned on SpeedDemosArchive for years before eventually being legalized.