Mages don’t spend all their time on Earth, and their gate magic can be adapted to allow for dimensional travel. Out of the other worlds that mages visit, the two most common types are bubbles and shadow realms.
A bubble is essentially a very small self-contained world, or (looked at from another perspective) a pocket universe. A bubble has no spatial connection to our reality, and as such cannot be accessed by any known means other than gate magic. Bubbles are typically very small, no more than a mile or two across, and often much smaller (though making a bubble too small brings its own problems).
The internal landscape of a bubble is highly variable, determined largely by its creator. Since a bubble is entirely detached from our reality, here’s nothing stopping a bubble’s creator from messing around with such things as a bubble’s atmosphere, strength and direction of gravity, heat and light, and so on. Once their creator starts using magic to modify the interior, the results can get really bizarre. There are stories of bubbles with impossible geometries: staircases that twist up, forwards, and sideways all at once, places where space warps in on itself so that you can see the back of your own head, and more. In practice, since bubbles are generally created by mages for their own use, the vast majority of bubbles tend to be recognisably Earthlike in appearance, or at least close enough that humans can exist there comfortably (though usually with one or two unusual features).
Lost and Found
The art of creating bubbles is very old. Bubbles were heavily used in the Precursor period but fell out of use after the Dark Wars, and the secrets of their construction were lost for over a thousand years before being independently rediscovered.
The modern technique for creating bubbles is laborious and time-consuming, and requires both the co-operation of a team of experienced mages and a significant expenditure of energy. The ritual channels an extreme amount of energy into a pocket of space: when the stress reaches a certain point, the section of space ‘tears off’ and loops in on itself, becoming separate. The disconnected pocket of space can then be reshaped using the standard methods.
Numerous mage historians have argued that this method probably isn’t how Precursor bubbles were created. The most common arguments in favour of this theory are that (a) surviving sources that reference Precursor bubble creation don’t make it sound anything like the modern technique, (b) the Precursors really shouldn’t have had the amount of power required to perform the (highly inefficient) modern method on a regular basis, and (c) modern bubbles have several distinctive features that don’t seem to match up with Precursor-made ones. Mages who believe this theory generally also believe that the Precursors either performed their bubble creation with outside help, or used some unknown and much more energy-efficient technique that’s yet to be rediscovered.
The major problem with bubbles is that once created, there’s nothing connecting them to our reality. They can only be accessed with gate magic, and such gate magic is significantly more difficult to perform than normal, usually requiring a focus. Every now and again gate magic to a bubble will fail, and once this happens that’s it – the bubble is gone forever, along with anything inside it. Possible theories for this phenomenon include interference between realities, the bubble being altered by some unknown force (thus disrupting the gate co-ordinates), the bubble being eradicated by some unknown force (thus giving no location for the gate to go to), or the bubble ‘floating off’ into the void. However, since there is currently no known method for reestablishing contact with a lost bubble, there’s no way for mages to discover which (if any) of these theories is correct.
The unreliability of bubbles has significantly limited their practical use. In the modern era bubbles have been largely superseded by shadow realms, and are only used when their inaccessibility is considered a selling point, generally for mages who are extremely security-conscious.