Collapse Pit

Pit Craters exist in volcanic regions across Mars, and form when surface materials collapse into large underground cavities. Such Pits are generally bowl-shaped, filled with sediment, and are relatively shallow when compared with their diameters.
Recently, a small number of anomalous pit craters were identified with strikingly different visible and thermal characteristics such as: sheer cliff walls; deep interiors that can extend out-of-sight beneath the Surface and temperature fluctuations that behave unlike any known feature on Mars.