Martian Caves (ctx frame - possible natural colors; credits: Lunexit)

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.