Craters are circular depressions caused by an extreme event such as volcanic eruption (volcanic craters), or asteroid collision (impact crater). Fictional craters are also frequently formed by magical explosions or superweapon detonation. As common elements in science fiction worlds, craters are frequently found on planets, moons, and asteroids, where they may be the location of planetary colonies or mining colonies. Craters are also often found in other genres, particularly as crater seas or crater lakes, where they can serve as interesting world locations. If a crater was formed by a collision, there may still be valuable space material at the center.