A storm shelter or storm cellar is a type of underground bunker designed to protect the occupants from violent severe weather, particularly tornadoes. They are most commonly seen in the US Midwest ("Tornado Alley") and Southeast where tornadoes are particularly frequent and the low water table permits underground structures.

A typical storm cellar for a single family would be built close enough to the home to allow quick access in an emergency, but not so close that the house could collapse on the door during a storm, trapping the occupants. This is also why the main door on most storm cellars is mounted at an angle rather than flush with the ground; an angled door allows for debris to blow up and over the door without blocking it and also decreases the force necessary to open the door if rubble has settled on top. It might have a floor area of eight by twelve feet (2.5 × 3.5 m) and an arched roof like that of a Quonset hut—but it would be entirely underground. In most cases the entire structure would be built of blocks faced with cement and rebar through the bricks for protection from the storm. Doing this makes it almost impossible for the bricks to fall apart. New ones sometimes are made of septic tanks that have been modified with a steel door and vents. Most storm cellars would be reached by a covered stairwell, and at the opposite end of the structure there would be conduits for air that would reach the surface, and perhaps a small window to serve as an emergency exit and also to provide some light. Storm cellars, when connected to the house, may potentially compromise security.