Water cycling during past wet episodes on Mars would have had many components. A thick atmosphere most likely carried a substantial amount of water evaporated from lakes and seas. That water vapour would, in turn, condense into clouds and eventually precipitate. Rain formed in this way would have created surface runoff, and much of this water would have percolated into the ground. Snowfalls might have accumulated to form glaciers, which in turn would have discharged their meltwaters into glacial lakes. Hydrothermal circulation, associated perhaps with sites of volcanism, could also have brought water to the surface from reservoirs deep underground.