Frost line
The depth to which the ground freezes in winter in your area.
The frost line (or frost depth) is how deep the soil freezes in a typical winter where you live — a few inches in the Deep South, four feet or more across the northern U.S. It's why water supply lines and footings are buried below it: anything above the frost line is exposed to freezing. Inside the house the same idea applies to where freeze risk lives — pipes in unconditioned space, against exterior walls, or near foundation penetrations are the ones that freeze first. See [how to thaw frozen pipes](/guides/how-to-prevent-and-treat-frozen-pipes).