Eero Saarinen won the 1947 design competition for a Jefferson National Expansion Memorial on the St. Louis riverfront with a sketch most jurors had never seen before in a public monument: an unadorned parabolic arch. By the time the engineering was finished it was not quite a parabola - it is a weighted catenary, the shape a chain would take if its links grew heavier toward the bottom, mathematically described by a hyperbolic cosine and refined to make the form look less pointed and more soaring. The cross-section is an equilateral triangle that tapers from fifty-four feet on a side at the base to seventeen feet at the apex. Each of the one hundred forty-two prefabricated stainless steel sections was filled with concrete and tensioned with steel rods. The arch is the largest single use of stainless steel in any construction project ever built.

Saarinen spent fourteen years refining the design after the competition - moving the arch onto higher ground, adjusting its proportions, fighting for the slender catenary shape against engineers who kept proposing something stockier. He never saw it built. He died of a brain tumor on September 1, 1961, at the age of fifty-one, just as excavation was beginning. Construction ran from February 1963 to October 1965 under the supervision of his successor firm and a consortium of structural engineers. The two legs were poured upward independently, with workers riding small derricks bolted directly to the curving steel. The closing keystone segment was lifted into the gap at the top on October 28, 1965, while Vice President Hubert Humphrey watched from a helicopter. The crews had to spray the south leg with fire hoses that morning to cool it - the sun was expanding it just enough to refuse the keystone.

The arch is hollow, and there is a tram inside. Nothing else in the world quite works the way it does. Eight egg-shaped capsules ride a track that climbs the inner curve of each leg, and because the track bends through more than ninety degrees of arc, each capsule is mounted on a ring that rotates as it rises, keeping the five passengers inside roughly upright. The cars were designed in the early 1960s and still feel like it - cramped, beige, faintly carnival. The ride takes about four minutes up and three minutes down. At the top, an observation room with sixteen small rectangular windows on each side looks east into Illinois and west across St. Louis. On a clear day the view reaches roughly thirty miles. The floor inside the apex is just wide enough for visitors to stand.

The arch was commissioned as a memorial to westward expansion and to Thomas Jefferson, who authorized the Louisiana Purchase and the Lewis and Clark Expedition. For the people who lived west of the Mississippi in 1804 - the Osage, the Lakota, the Pawnee, the Comanche, the Shoshone, the Nez Perce, and dozens of other nations - that expansion did not feel like an opening. It meant treaties broken, lands seized, populations displaced and decimated. The original museum beneath the arch leaned heavily into the heroic frontier story. The current Museum at the Gateway Arch, redesigned and reopened in 2018, broadens the frame: it places Indigenous nations at the center of the western half of the story, treats Lewis and Clark as travelers through inhabited territory, and locates the Old Courthouse on the same campus, where the Dred Scott case began. The arch still says gateway. The museum underneath says it was also a closing door.

Gateway Arch National Park covers about ninety-one acres on the west bank of the Mississippi, from the Old Courthouse on Fourth Street down a long landscaped slope to the river. The grounds were redesigned along with the museum: a continuous green roof now connects downtown to the riverfront, eliminating the elevated highway that used to cut the monument off from the city. The arch is climbed by elevator and tram, not by stairs - there is no public route up the outside. At night, since 2001, forty-four floodlights in below-grade pits bathe the steel in white from ten p.m. to one a.m. The reflection on the river doubles the curve. From the Eads Bridge, a few blocks north, the geometry is at its cleanest: river, arch, sky.