Water ice buried at Mars' equator is over 2 miles thick 

 The Mars Express orbiter has detected enough water ice buried beneath the Red Planet's equator to cover the entire planet in a shallow ocean if melted. 

https://cdn.mos.cms.futurecdn.net/REpvNAHQnZJbXWfBqPEy8h.jpg

https://www.space.com/mars-water-ice-equator-frozen-ocean