This chapter talks about the astronomers that have solved some of the riddles of how much water Mars had and still has, a century and a half after William Huggins first proved that Mars had water. It highlights that Mars' water was no longer in the liquid state on the surface as the planet had become much dryer after billions of years. It also examines significant evidence that strongly suggests that Mars experienced a more recent epoch when water carved valleys and was formed from melting snow and ice that flowed slowly into and out of chains of lakes. The chapter describes one of Mars' lakes that appears to have been widespread both north and south of the Martian equator, containing more water than Lake Ontario. It discusses Mars' layered ice deposits at the north and south polar caps, in which a thin layer is deposited each Martian winter and then sublimates in Martian spring.