Multivarium

The Multivarium was, as the Second Doctor described it, "created at the intersection of parallel universes". He suggested it was a lesson or a prison, but didn't know for sure. Anything and nothing was possible, and it was impossible to enter or leave. It consisted of white rooms and doors of all shapes and sizes, some with pure oxygen, or argon, or nitrous oxide, and some with fire or clouds or dust or rain or extra gravity. The Doctor and another person spent years trapped in the Multivarium, without becoming hungry or tired. Eventually, the Doctor realised that because anything was possible in the Multivarium, it could be impossible to leave in one room, but possible in another, and used this to get out. (PROSE: The Room with All the Doors)