Viewed from space, the twin gates of the Maeslantkering—the largest movable objects ever built—looked a bit like the pie-wedge–shaped wings that children scraped out in fresh snow when they were making angels. The curved rim of each pie wedge was the actual barrier meant to stand fast against the full force of a North Sea storm surge. Each was made to block one half of the width of the waterway joining Rotterdam to the sea. They came together in the middle, each swinging inward from a pivot on the embankment. The rest of the pie wedge—the triangle that spanned the 240-meter radius from the curved barrier to the pivot—consisted of massive steel tubes welded together to form a rigid trusswork. That structure transmitted the sea’s force from the barrier to the ball and socket that might be thought of as the shoulder joint of the angel’s wing. It went without saying that these were the largest ball-and-socket joints ever made. The balls were ten meters in diameter. They were cradled in sockets consisting mostly of reinforced concrete, set deeply into the ground and distributing the force into the surrounding earth through long concrete buttresses that splayed out like fingers and eventually ramped down below ground level to connect with massive subterranean footings.