This was first discovered by the noble Arcturus Rookou I, who never really made much use of the space, being so busy with his 'Secret Serpent Sanctuary'. This particular part of it can be accessed by a tunnel in Caecey's wing, only when you know the password. Not many people know about this, because the hole opens in the bottom of Caecey's swimming pool. The password can be reset in the Control Room, by Caecey herself. The Underground is surveyed by magical cameras hidden in the walls.
Arcturus, if he so wishes, can hack into Caecey's system, although he has not yet tried this.
Most of these modern contraptions were added by Caecey, seeing whether she could make use of the space. It is like some sort of Headquarters, designed by Caecey a year ago, before Ciaran's departure, as to make something against what she calls "Mudbloods", the average nickname used by Muggle-haters to describe, as I must crudely call it, "Magical Muggles" e.g. people with no wizarding family that can use magic, see Hogwarts, hold a wand etc.