The Tharsis region is thousands of kilometers across and averages nearly 10 km (33,000 ft) above the mean elevation of the planet.

They were among the few surface features visible as the spacecraft entered orbit during a global dust storm.

A fourth spot corresponding to the albedo feature Nix Olympica was also visible and later named Olympus Mons.

As the dust cleared, it became obvious that the spots were the tops of enormous shield volcanoes with complex central calderas (collapse craters).

The three volcanoes, most notably Arsia Mons,[7] also all have collapse features and rifts, from which flank eruptions issued, that transect them along the same northeast-southwest trend.