Geological Chronicle
Oceanus Procellarum is the largest volcanic plain on the Moon, covering roughly 4.0 million square kilometers across the western near side. Stretching 2,500 kilometers north to south, this single dark expanse accounts for 10.5 percent of the lunar surface. Because of its exceptional size, 17th-century astronomers designated the region an ocean rather than a sea. Unlike circular maria such as Mare Imbrium or Mare Crisium, Procellarum lacks a continuous mountainous outer ring.
Data gathered in 2014 by NASA's GRAIL gravity mission showed that Procellarum is bordered by a quasi-rectangular system of ancient rift valleys buried under the lava plains. These steep graben structures formed when subsurface magma upwelled and stretched the brittle crust, causing it to fracture along linear faults rather than collapsing into a circular impact basin.
Procellarum sits within the Procellarum KREEP Terrane (PKT), an anomalous crustal province enriched in potassium (K), rare earth elements (REE), and phosphorus (P), along with heat-generating thorium and uranium. This radiogenic heating kept the underlying mantle molten far longer than elsewhere on the Moon. Volcanic activity created complex landforms across the plain, including the Marius Hills, the Moon's densest concentration of volcanic domes and sinuous rilles, alongside prominent ray systems from Copernicus and Aristarchus craters.
The broad, low-slope terrain made Procellarum a primary target for lunar exploration. In 1966, the Soviet probe Luna 9 and NASA's Surveyor 1 completed humanity's first robotic soft landings here. In November 1969, Apollo 12 astronauts Pete Conrad and Alan Bean landed the Lunar Module Intrepid within 165 meters of Surveyor 3 at Statio Cognitum. In December 2020, China's Chang'e 5 mission landed near Mons Rümker and returned 1.73 kilograms of basalt dated to 2.0 billion years ago, confirming that lunar volcanism persisted into the Eratosthenian period.