Spacecraft circling the Moon in the 1960s kept drifting off course, and the reason turned out to be buried concentrations of mass under the big round seas.

They were named mascons, for mass concentrations, and they are dense material that welled up under the impact basins. They pull a passing spacecraft down by enough to matter, which is why most low lunar orbits decay in months and why picking a stable one is a real piece of work.