A communication vault, built much like a burial vault (seriously, it's usually one company that casts both). It is made of thick concrete, buried underground, and (with a water intrusion alarm) fairly well protected from above-ground ills.
Pretty much, but the one Commcast was dropping in had an AC power feed, a set of lead acid battery boxes built into one side, and a moulded in space for holding about 40U of rack gear (telcom depth though so it looked like 12" not 40". Oh and a ladder in one side and a lockable lid. Very interesting kind of box.
We called those "CEVs", or Controlled Environment Vaults when I worked for the phone company. I felt like a secret agent the first time I went into one to build a T1 circuit. Just an unassuming lid at street level, but when you climb down the ladder (about 16 feet down to the bottom) you're in a 12' x 24~40' lair‡, with racks and cable frames, batteries, a little desk with an ancient teletype on it, some switch gear, etc. A whole secret world that most people don't know about. And these things are all over the place!
Nice to put a name to it, it was definitely a CEV. It wasn't quite that large but Googling around I can see that you've correctly identified it from my description.
