It's pretty obvious what it is. The feature rises prominently above the Martian terrain. Because it is located south of Ascraeus Mons - a large volcano within the Tharsis volcanic plateau on Mars - NASA scientists believe it was formed by volcanic processes.
But you know, and I know, that this is a geoglyph, proving that ancient Martians worshipped rubber duckies.
More information at NASA's Mars Exploration Program.