He took the elevator, descending two floors underground to a small, claustrophobic room - the vault antechamber. A 3-ton steel vault door dominated the far wall. It alone had six layers of security. There was a combination wheel with numbers from 0 to 99. To enter, four numbers had to be dialed, and the digits could be seen only through a small lens on the top of the wheel. There were 100 million possible combinations.
The door was monitored by a pair of abutting metal plates, one on the door itself and one on the wall just to the right. When armed, the plates formed a magnetic field. If the door were opened, the field would break, triggering an alarm. To disarm the field, a code had to be typed into a nearby keypad. Finally, the lock required an almost-impossible-to-duplicate foot-long key.
Edit: Let me get this straight... they put the magnetic "door open" sensor on the outside of the impenetrable vault door? You can almost forgive the lazy guard who keeps the supar-sekrit key in the nearby utility closet, but installing an alarm on the outside of the vault, where anyone can tamper with it?? That's an awfully stupid mistake for an installer who's supposed to be an expert on alarms.
Edit 2: More articles by the same author at his website.