The Department of Mysteries is a section of the Ministry of Magic that carries out confidential research. Most of its operations are carried out in total secrecy. Few wizards within the Ministry actually know what is located within this department, though it is learned that the various mysteries of the world are studied there, these include: love, space, thought, time, death, and others. Those wizards who work in the Department of Mysteries are known as Unspeakables because of the confidential nature of their work.

Due to the highly classified nature of this department, it is granted a great deal of independence, being the only one within the entire Ministry that does not need to answer to the Department of Magical Law Enforcement. Even the Minister for Magic has very little authority over the department's operation.

The Department of Mysteries lies on the second-lowest level of the Ministry of Magic, Level Nine. It is accessible via the lifts from the Ministry Atrium. This level bears a striking difference to those above: the black-tiled walls are bare with no windows and no doors, apart from a plain black one at the end of the corridor that leads into the Department proper. Light is provided only by torches, which glow with a blue-white light. A small flight of steps to the left leads to Level Ten.

Behind the black door is the Entrance Chamber, which is designed to disorient any unauthorised personnel who enter it. It is a circular room with a dark marble floor that looks almost like standing water, candles emitting a cool blue light, and twelve handleless doors. Whenever a door closes, the walls rotate, making it impossible to determine which door is which. This chamber will respond to a verbal request for an exit by the opening of the correct door.

Divisions: Brain Room, Death Chamber, Love Chamber, Space Chamber, Time Room.