Around here, depending on the time of the year, people can put in a fiberglass septic tank and not put water in it to weight it down, the next morning it will be popped up out of the ground.
Same with swimming pools. Drain them completely, leave it over night, and it’s a good bet it will have floated up out of the ground a foot or two the next morning.
That’s a bad thing to find at your construction site when you get there in the morning.