A mountain on Mars that is almost as tall as Denali in Alaska appears to be surprisingly light, scientists reported on Thursday.

For more than four years, NASA’s Curiosity rover has been exploring Mount Sharp, located within an ancient meteor impact crater known as Gale and rising more than three miles high . Now measurements of tiny changes in gravity, recorded by the rover as it climbed in elevation, could help solve the question of how the mountain formed.

The official name of the mountain is Aeolis Mons, but mission scientists have nicknamed it after Robert P. Sharp, a Mars expert who died in 2004. It consists of layer upon layer of sedimentary rock, which offer an easy-to-read history book of Martian geology. That was one of the attractions that led NASA to choose the site for the Curiosity mission, which landed on Mars in 2012.

But sedimentary rocks typically form at the bottoms of lakes and oceans, not at the tops of mountains.