My pipes are not burst!
I crawled under the house, and was immediately wary of a pool of water I found accompanying the sound of running water.
On closer inspect, I saw that water was coming into the crawl space from outside and pouring into an open catch-all drain pipe, so the water was in a limited place (whoever built this house actually did something cool that really paid off).
I went outside and lo and behold found a vacuum something breaker for what I'm guessing is the sprinkler system had popped off the water head and broken, and was gushing water.
I opened the green access hatch below and shut off the water, and it stopped, and mysteriously the rest of the faucets in the house came on at full blast. There must still be another master water valve upstream, note to self, find it.
I'm heading off to the home improvement store to buy a new plastic vacuum breaker thing.