You're right - the hole is a 3 storey building located 1.5 miles underground. It's just a regular office building, except no windows, obviously. You get on a bus and ride underground, and the building is built in a cavernous area. It's actually pretty cool to see. There is a water supply and air pump system so that in the event of a nuclear attack, the hole could be completely sealed off and there would be enough water and air to keep people alive for awhile. I'm not sure exactly how long 'awhile' is, but if there was a nuclear attack, I'm not sure I'd be too keen on making it out of the hole anyway.
It was built before the Cold War. Since the Cold War is over, the threat of nuclear attack went away. It's quite expensive to run the UGC, so that's my understanding of why it's being closed down.
So now they have built a new building above ground to perform the same operations.