Volcanoes on Mars that involve water-logged sediments (mud) rising to the surface. Several have been studied at Acidalia Planitia. These volcanoes cool down more quickly at night than the surrounding area. The mounds are more oxidized than the surrounding plains, which could be Iron Oxides, which form in liquid water. They could have formed in the last 10 Million Years. They are thought to be a good place to search for evidence of life because it may be warm enough deep underground for water to remain liquid. Mud volcanoes would bring up any such water, along with any microbes. It is possible that these could be a source of Methane gas, though their methane content has not been measured.