A recent study by Purdue University researchers has revealed the presence of many hidden geological formations just under the Moon’s surface. The researchers discovered these craters after analyzing data collected by NASA’s GRAIL (Gravity Recovery and Interior Laboratory) mission. After sifting through lunar gravitational measurements, researchers detected several ring-shaped gravitational anomalies which showed slightly stronger gravitational pull. These imply the presence of massive underground craters which have been covered up with lunar lava and soils since their formation billions of years ago.
According to Purdue University geophysicist Henry Melosh, the GRAIL data is like a...well, grail of data about lunar composition and history:
Those gravity variations have been known for some time, but not with the precision we have now. If you were walking around on the surface of the moon, you probably wouldn't notice it. But if you had a pendulum clock, you'd probably see that it was running faster inside the basin.
One of the newly discovered lunar craters measures 200 kilometers (120 miles) in diameter and has been named after Amelia Earhart. Another nearby hidden crater is believed to be roughly 160 kilometers across (100 miles) and has been dubbed the Ashoka Anomaly.
According to their preprint study of the craters set to be published in Icarus, the researchers believe there are many more undiscovered hidden craters dotting the lunar surface, implying a much richer geological history than the Moon's surface would have us believe:
The presence of buried craters is further supported by individual analysis of regional free-air gravity anomalies [...] Other large, still unrecognized, craters undoubtedly underlie other portions of the Moon’s vast mare lavas.
Because an object's gravity is determined by its mass, empty pockets such as craters or underground caverns can cause small but measurable fluctuations in gravity. Several of these gravitational anomalies exist on Earth. These craters and other recent discoveries about the Moon’s formation are showing that we perhaps don’t know as much about our closest cosmic neighbor as we once thought.