Mars once had enough water for a global ocean several hundred metres deep, here’s an interesting on article on where the water might have gone:
“Mars is losing little water to space, according to new research, so much of its ancient abundance may still be hidden beneath the surface. Dried up riverbeds and other evidence imply that Mars once had enough water to fill a global ocean more than 600 metres deep, together with a thick atmosphere of carbon dioxide that kept the planet warm enough for the water to be liquid. But the planet is now very dry and has a thin atmosphere.
Either some other process removed the water and CO2 or they are still present and hidden somewhere on Mars, probably underground, Barabash says. “We are talking about huge amounts of water,” he told New Scientist. “To store it somewhere requires a really big, huge reservoir.”
Barabash is not sure what form this reservoir – or reservoirs – would take, but he points to findings from NASA’s now lost Mars Global Surveyor. This data provided evidence that water had gushed down slopes on Mars in recent years, possibly originating from beneath the surface.”