At a house next to the ocean, the last two “floods” were:
1. Safety pressure valve on top of 60 gallon electric hot water heater went bad and started spray pulsing out water despite water pressure being normal.
2. Eleven years before that, copper water pipe broke in bedroom bathroom.
About $8000 of damage each time.

Another time I casually walked outside by the electrical meter and was shocked to see a stream of clear water literally shooting out the electrical conduit from below the meter where it made an elbow to go under the concrete slab the house sits on.

Turning off the main water valve for the house stopped this stream of clear water.

I eventually found that when the house was built in 1953 they had laid the electrical conduit on top of a copper water pipe, and galvanic corrosion slowly created a hole in both the steel and copper.

To find this spot I hired an “Ultrasonic Leak Detector” company to pinpoint the spot in the floor of the house using triple microphones with low frequency sounds first ( low freq travels farther) and subsequently higher and higher frequency sounds that could only be detected very close to the leak. I then used a hammer drill to make a 12 inch circle of holes in the concrete floor at this suspected spot, then drilled between the first drilled set of holes with larger diameter holes to intersect and lift the circle out. Leak was within 3 inches of where the leak detection guys predicted.

Soldered a repair and refilled the circle with urethane instead of concrete. I still worry that somewhere beneath the house slab there is at least one more spot where the original construction crew let copper water pipe touch steel electrical conduit.