Prior to the mission, they were thought to be volcanic plains that are comparable in age to the Imbrium impact basin. The Apollo 16 crew collected 731 individual rock and soil samples, including a deep drill core that included material from 2.2 meters below the Moon's surface, with a total mass of 96 kilograms. These samples provided the biggest scientific surprise of the entire Apollo program. Although it was thought that this region of the Moon was covered by volcanic plains, this turned out not to be the case. Instead, virtually every rock collected on Apollo 16 was a breccia.
Breccias are rocks that are composed of fragments of other, older rocks.
Over its long history, the Moon has been bombarded by countless meteorites. These impacts have broken many rocks up into small fragments. Sometimes, the heat and pressure of these impacts fuses small rock fragments into new rocks, called breccias. Many such fragments are visible in the breccia shown above. In some cases, the rock fragments that form a breccia are themselves even older breccias.