Nasa has unveiled an amazing simulation of the wet, warm planet that Mars used to be - before it all went wrong.
The artists' impressions video captures the look and feel of the planet, with rolling white clouds and lakes of liquid water on the surface.
Mars today has no liquid water on the surface, due to low atmospheric pressure and its cold temperature.
But around four billion years ago it was a different story. Nasa's probes on the surface have so far turned up evidence for a world rich with the conditions in which life could emerge.
Joseph Grebowsky of Nasa's Goddard Space Flight Centre said:
"There are characteristic dendritic structured channels that, like on Earth, are consistent with surface erosion by water flows. The interiors of some impact craters have basins suggesting crater lakes, with many showing connecting channels consistent with water flows into and out of the crater. Small impact craters have been removed with time and larger craters show signs of erosion by water before 3.7 billion years ago. And sedimentary layering is seen on valley walls. Minerals are present on the surface that can only be produced in the presence of liquid water, e.g., hematite and clays."
The video - released to promote the work of its new probe Maven, which will be launched in November and arrive in orbit around Mars in 2014 - shows the transition to the dusty, red world we know today.
There are several theories about how Mars was stripped of its atmosphere. They include the possibility of large-scale asteroid strikes, and the loss of its "intrinsic magnetic field" which protected it from erosion by the solar wind.