The boys woke us up with 5 AM kitten soccer, but it gets better.
From the system log of our mail server (and this web server):
Jul 7 07:46:34 diana apcupsd: Power failure.
Jul 7 07:46:36 diana apcupsd: Running on UPS batteries.
Katherine hears beeping, finds basement door open, UPS unplugged. She plugs UPS back in, secures door.
Jul 7 08:00:17 diana apcupsd: Mains returned. No longer on UPS batteries.
Jul 7 08:00:17 diana apcupsd: Power is back. UPS running on mains.
That particular UPS cord plugs into the ceiling and doesn't have much slack, which means a cat could trip over it and pull it out with his weight.
(The cord dropping out of the sky and the sudden beeping were no doubt their own negative feedback to the feline engineer who did this.)