Chaotic terrains visible on the Martian surface today indicate a complex history of transport, storage, and release of large amounts of water in the past. Scientists believe these areas formed by collapse of a surface overlying a reservoir containing ice and sediments, when the ice melted due to, e.g., a volcanic or impact heat source. After drainage of the water masses, broken surface blocks remained in the cavity that previously hosted ice and sediments. Possibly, groundwater as well as magma upwelling could have occurred during collapse in addition to the release of meltwater. For Aurorae Chaos, crater size-frequency distribution measurements yielded a model age of 3.5 billion years for the basin floor, indicating collapse happened long time ago.