Ever since Galileo's telescope made the rugged lunar surface more clearly visible (in 1610), men have strived to learn more about the origin and history of the Earth's big natural satellite, and never has so much progress been made as in the last few years. The fifth manned lunar landing was in a highlands area, quite different from the sites visited previously, and the discoveries there now seem certain to result in significant improvements in the hypotheses of lunar scientists. Much of the Moon's surface is similar to the Descartes Highlands that the Apollo 16 astronauts examined. From this highly productive mission, more photographs were obtained than on any previous Apollo flight, a greater amount of time was spent outside the lunar module, a greater weight of scientific equipment landed on the Moon, and a record weight of scientific samples was brought back to laboratories on Earth. The network of automatic scientific stations at work on the Moon was extended into a new area and has since detected a moonquake caused by the largest meteoroid impact that has yet been recorded. Additional experiments on the surface and in flight also were successfully performed on this mission for the enlightenment of students of natural phenomena. The Apollo 16 astronauts observed, and scientists studying material they collected have subsequently deduced, that this landing site differed surprisingly from earlier expectations. Future generations consequently may benefit from better concepts of the operation of the solar system and events throughout the physical universe than have hitherto been possible. This volume is but one of a series of NASA Special Publications being issued promptly to document potentially significant discoveries in the course of the Apollo Program, thereby possibly increasing their usefulness to scientists grappling with problems that have long perplexed mankind. In most difficult endeavors, experience engenders both confidence and skill. The manned exploration of the Moon is no exception. The reaching of those parts of the Moon that contain both the oldest and most intriguing rocks had to await acquisition of such experience. The character of the relatively smooth expanses of the lunar maria was established by the samples and results of experiments from the Apollo 11, 12, and 15 missions. The exploration of the older lunar terra began with the Apollo 14 and 15 missions. The Apollo 15 orbital-science results and the detailed study of soils from the Luna 20, Apollo 11, and Apollo 14 sites demonstrated that the terra materials in the Fra Mauro and Hadley regions do not exemplify most of the lunar terra. Geochemical studies indicate that the typical terra must be underlain by rocks richer in aluminum and poorer in the radioactive elements uranium, thorium, and potassium than the samples from the Hadley and Fra Mauro sites. The characterization of the aluminum-rich highlands, which make up more than four-fifths of the lunar surface, became the major objective of the last two Apollo missions as soon as the Apollo 15 results were understood. The ultimate origin of the aluminum- or plagioclase-rich terra rocks is a secret that may be buried under a thick debris that was produced by an intense bombardment of the lunar surface before the formation of the mare surfaces. It was thought that the accessibility of the primitive rocks was complicated further by younger igneous rocks that intruded and covered the early aluminum-rich crust in many parts of the terra. The investigation of such igneous rocks, along with a search for samples of the primitive aluminum-rich crust, was the prime objective of the Apollo 16 mission.