Sample of lunar KREEP, with a mass of 82 grams, collected by Apollo 12. Scale shown is in cm. Image: NASA Johnson Space Center.

KREEP is a lunar basaltic rock, rich in the radioactive elements uranium and thorium. It is believed to have formed early in the history of the Moon during the solidification of the Moon's molten stage, known as the magma ocean, and is now found in the lunar highlands. The acronym comes from K for potassium, REE for rare-earth elements, an P for phosphorus. Only one sample of KREEP was returned by Apollo 12 (the one shown here), but many additional samples were collected on Apollo 14 and Apollo 15.