Large rectangular feature below lunar surface indicates tectonic activity

BBC : Until 2012, when NASA’s GRAIL mission mapped the Moon’s gravity gradients, it was assumed that the Procellarum—a large lunar mare on the Moon’s near side—had been formed as the result of an ancient giant impact with a smaller celestial body. However, by looking more closely at variations in the pull of gravity over the Moon’s surface, researchers were able to detect an excess of mass that they attributed to the presence of basaltic lava filling rift valleys. Because of the presence of large amounts of uranium, thorium, and potassium in the Procellarum region, they propose that early in the Moon’s existence those radioactive elements would have caused the crust to heat up and expand. Subsequent cooling would have then caused the surface to contract and led to the creation of large rifts, which later were filled in by volcanic lava. The theory would explain the Procellarum’s distinct rectangular shape, as opposed to the more circular shapes of other lunar maria.