On August 1, 1971, while exploring the eastern edge of the lava plain known as Mare Imbrium on the silent, serene lunar surface, Apollo 15 astronauts David Scott and James Irwin found something remarkable: a profoundly old piece of lunar crust, a relic more than four billion years old that carried clues to the moon’s formation. Seeing the glint of ancient crystals embedded in what would later be called the Genesis rock, Scott immediately knew its potential importance for solving the mystery of how the moon was made. “I think we found what we came for,” he radioed to mission control as he and Irwin retrieved the rock and placed it in a bag. It would become a key part of what is the Apollo program’s greatest scientific legacy.