Bunkers are buildings designed to protect their inhabitants from an outside danger (super storms, war, doomsday scenarios, etc.). Bunkers may contain storage areas for food, water purification systems, ventilation shafts, entertainment centers, hydroponic gardens and more. They may have hidden entrances and exits, airlocks, or decontamination chambers. They may be built underground or within mountains. If designed for a large enough population, bunkers may resemble underground cities.

When designing bunkers consider how they protect and sustain their users. Bunkers designed for long-term habitation may have areas designed for fitness, entertainment, food production, air recycling, and communications.

Subways, cellars, and other underground elements may be used as makeshift bunkers.