Atlantis is a submerged city, a charged crystal pushes the water back and made it accessible to humans,
The crystal got overcharged and exploded. The result was a thick, rich atmosphere that nourished giants that lived for 200 odd years. The atmosphere thinned down since, and the human got shorter and smaller.
So, to create a thick atmosphere on Mars overcharge a crystal where there is water or ice. In a controlled manner...
