The Vault (Tales from the Vault)

The Vault was a location where UNIT kept various alien artefacts, as well as records of their capture. Both Captain Ruth Matheson and Charlie Sato worked there. The Vault was permanently kept dark, as some objects would react with light. It was known to contain a deactivated Terravore, Krynoid pods, possibly the wreckage from a Sontaran scout ship, an army jacket, a painting, crystal and a wax cylinder. (AUDIO: Tales from the Vault)

Excalibur was held in the Vault. The Vault was also where the Master was imprisoned when he was captured by UNIT following his escape from the Eye of Harmony. (AUDIO: Mastermind)

Although similar in intent and with similar safety precautions, this was a different facility than the Black Archive also maintained by UNIT. (TV: The Day of the Doctor)

Behind the scenes
When Gary Russell tweeted Mastermind writer Jonathan Morris, asking him if the Vault in Mastermind was the same as the one from Russell's novels, Morris responded, "Possibly, but it's now been moved to a secret base under [the] Angel of the North". When he realised Russell's C19 Vault was a short distance from the one in Mastermind, Morris stated that "it all fits!"