Go through a door in our laundry room and you're outside, at the bottom of a short flight of stairs. Also at the bottom of these stairs is a drain. The drain got clogged, the water went under the door into the laundry room. The water probably found the drain in the floor of this laundry room and headed that way, until it, too, got clogged. With no way to enter the sewer pipes, the water started looking for other places to go. The easiest route, it seems, is into the main (finished, carpeted) area of the basement. Ugh.
The water seeped easily through the carpet, spreading about 10 feet from the laundry room doorway. Once it found its way to the uncarpeted floors of the bathroom and furnace room, it spread more quickly.
The Great Flood of 2000 was caused by a leaky washing machine hose and compounded by lots of stuff sitting on the floor and exacerbated by our being out of town for five days. We'd (mostly) learned our lessons from that flood, so property damage from this flood is negligible. ServiceMaster came on Monday and lifted the carpeting, removed the pad, and left a bunch of floor fans blowing from the underside of the carpet. They'll return in about a week to put down new padding and wash the carpet. Then we can set up the basement again and prepare for the next flood.