The Apollo astronauts Charles Duke and John Young train on the geological rover, or "Grover," a lunar rover trainer, in Arizona. Both Duke and Young went on to walk on the surface of the moon in 1972, during Apollo 16. #

Explosions send dirt into the sky during the construction of Cinder Lake Crater Field #2 on July 27, 1968. A total of 354 craters were carved out of the Arizona landscape in this field to create a simulated lunar surface. #

A post-explosion aerial view of the completed Cinder Lake Crater Field #2, showing very light ejecta caused by excavation of clay beds immediately below black basaltic cinders. #

The astronauts Harrison Schmitt and Gene Cernan view a crater within the Cinder Lake Crater Field. Both men went on to walk on the surface of the moon in 1972, during Apollo 17. #

The USGS geologist Joe O'Connor wears an early version of the Apollo spacesuit during testing in the fall of 1965, at Apollo mesa dike in the Hopi Buttes volcanic field in Arizona. #

A suited subject stands on the edge of the Bonito Lava Flow near Arizona's Sunset Crater volcano, with an early concept of a "lunar staff" with a sun compass on top. #

The astronauts Jim Irwin and Dave Scott check out an outcrop during the final geology exercise for the Apollo 15 prime and backup crew at Coconino Point, Arizona, on June 25, 1971. Both men went on to walk on the surface of the moon in 1971, during Apollo 15. #

The Apollo 17 astronaut Harrison Schmitt stands on the edge of an erosional canyon that was used to simulate the large Hadley Rille at the Apollo 15 landing site. Schmitt was originally part of the Apollo 15 backup crew, and later slated to fly in Apollo 18. When that mission was canceled, Schmitt was moved up to Apollo 17 due to his previous work and studies as a professional geologist. #

The USGS geologists Tim Hait and David Schleicher work in spacesuits during an early Apollo field test at Cinder Lake Crater Field #1, with a simulated lunar-module ascent stage mock-up parked on an earthen ramp in the background. #

The hand-constructed Explorer vehicle, made by the astrogeology branch of USGS, is field tested in June 1967 on an extremely blocky lava flow just west of Hank's Trading Post, near Flagstaff, Arizona. #

A modern satellite view, made in 2011, of Cinder Lake Crater Field #2, north of Flagstaff, Arizona, decades after its use as a lunar training location. See it mapped. #