A low-density, weak-gravity region has been found below Olympus Mons and the Tharsis volcanoes, while Mars' northern hemisphere is littered with puzzling high-gravity structures beneath the surface.

The largest volcano on Mars may sit above a 1,000-mile magma pool. Could Olympus Mons erupt again?

Source: space
Source Link: https://www.space.com/could-newly-discovered-magma-plume-cause-oympus-mons-to-erupt-after-millions-of-years