I went down to check on laundry last night, to find running water on the basement floor, and all over the unused downstairs bathroom.
Our house was two apartments that we combined to one. I'm working on getting them back to two so I can rent downstairs. My latest project is doing the necessary utilities to move the washer and dryer to the attic. In the meantime, they are still in the basement.
It turns out that the hot water pipe for the tub in the bathroom broke. Someone used a brass elbow to transition between copper and galvanized. The galvanized was very corroded.
I don't know if it froze, or if the corroded area let go on its own, either are possibilities. I think it froze, because there's also part of the valve that has broken.
I turned off the hot water to the downstairs which slowed the rushing water to a trickle, but didn't stop it. No other valves except the whole-house valve, so I shut that off. I was able to find a plug, cut the offending pipe and plug the end at midnight so I could turn the water back on to the rest of the house.
...except this morning, there was still a slow drip from the cold side of the tub valve. That area looks corroded too, and there is evidence that something in the area has been leaking for a while--More than likely what I thought was splashing from the tub damaging the floor was actually that leak. Cut the supply pipe to that area as well, plugged the end, and it seems to be drying out now.
We have a partial basement, and the leak was over crawlspace. Luckily it is a fairly deep crawlspace. Unluckily, the leak made a nice bed of mud, and there are years of odd junk that have been tossed into the crawlspace. I think I'm going to put off repairs until that drys out.