The Keystone of the Roman Arch

In 19 BC, Marcus Agrippa oversaw the construction of the original Pantheon in Rome. Roman engineers understood something remarkable about the arch: every stone leans inward, pressing against its neighbors, each one ready to collapse without the others. But there is one stone — the keystone at the top — that makes the entire structure possible. Remove it, and the whole arch crumbles into rubble.…