Olympus Mons is a shield volcano. Geologist think it took at least a billion years to get so big, and they think it stopped erupting about 1 billion years ago. There are three other shield volcanoes near Olympus Mons, they are Ascraeus Mons, Povnis Mons, and Arsia Mons. If there was running water on Mars there would have to be a thicker atmosphere. There would also have to be temperatures above freezing level.
Scientists think that the water that was on Mars evaporated into space.
Scientist also think that there might be water frozen under the surface of Mars.