Mars‘s crusty interior may harbor massive reservoirs of water, enough to fill an ocean, according to a recent study of seismic data.

Scientists have long though that Mars’s surface was once covered by water, most likely stretching across at least one third of the terrain in a massive northern ocean. But the water dried up a millennia ago, and those studying the red planet have debated whether the water simply evaporated due to the planet’s size or was absorbed beneath the surface to be held in vast underground reservoirs.