Mysterious Portal

Mysterious Portal is the name given to many initially-unknown portals to a different dimension, especially dimensions otherwise hard to reach.

How they work
It is currently unknown exactly how portals like these operate. What is known is that the journey takes roughly four hours of normal time, but this may differ in certain dimensions in which time flows differently. This delay in arrival appears to the traveller to be so miniscule as to be instant. There are only six known portals in existence which can send matter between planes (excluding material planes three and four).

Locations
A list of known locations of Mysterious Portals can be found here.