|The following is based on Fallout: Brotherhood of Steel and is not canon.|
It was likely discovered by Rhombus while seeking a main base of operation for the Texas expedition against Attis and his super mutants. The place appears to be a Vault-Tec facility, that may have been dedicated to the Secret Vault project.
Some valuable technology and inventory, like working computers, laser grids, functional steam pipes, radioactive drums, gun bots, and holographic devices were found there and used for training new initiates.
In the game, Nadia completes this course, guided by a paladin hologram, while various people in vault uniforms watch her progress. These people may be the Brotherhood of Steel members wearing uniforms found in this location.
The vault prototype appears only in Fallout: Brotherhood of Steel.