Finnish architect Eero Saarinen won the 1948 design competition for a memorial to westward expansion with a radical concept: a stainless steel catenary arch - the shape formed by a chain hanging between two points, inverted. At 630 feet tall (and 630 feet wide at the base), it would be the tallest man-made monument in the United States. Saarinen died in 1961, four years before construction began; his partners completed the engineering. The arch's shape wasn't just aesthetic - the catenary curve transfers all loads through the structure as compression, making an impossibly slender form remarkably stable.

Construction began in 1963 with separate crews building each leg. The triangular sections were fabricated in Pennsylvania, shipped to St. Louis, and assembled using specialized derricks that climbed the growing arch. Each section was double-walled stainless steel, with interior carbon steel shells and concrete filling for weight and stability. Tolerances were measured in 64ths of an inch. As the legs rose and curved toward each other, workers rode in the creeper derricks 600 feet above the Mississippi, setting sections in position with precision that seemed impossible at the time. And might still.

The final section - the keystone - had to fit between the two legs after they'd risen independently through two years of construction. Engineers knew the steel would expand and contract with temperature; they planned for final assembly on a cool morning when the southern leg (in full sun) would expand enough to provide clearance. On October 28, 1965, the northern leg was jacked outward, the 8-ton keystone was lifted between the legs, and fire trucks sprayed water on the southern leg to control expansion. The section dropped into place. The arch was complete. No workers died during construction.

Visitors reach the observation deck via a tram system - small capsules that travel through the arch's hollow interior, rotating to remain level during the curved ascent. The ride takes four minutes; the view from the top encompasses 30 miles on clear days. The Museum of Westward Expansion beneath the arch interprets the Lewis and Clark expedition and subsequent migration. The grounds are part of the Gateway Arch National Park, connected to the St. Louis riverfront. The arch gleams in sun and glows in artificial light; it's become St. Louis's symbol, visible from much of the metropolitan area.

Gateway Arch National Park is located on the St. Louis riverfront, accessible from downtown. Tram rides to the observation deck require timed tickets (book in advance for popular dates). The museum beneath the arch is free and worth visiting. The grounds offer views of the arch and the Mississippi River. Riverboat cruises depart from the adjacent levee. The Old Courthouse, site of the Dred Scott case, is part of the park. St. Louis has extensive lodging and dining nearby. Visit early morning or sunset for the best light on the stainless steel; visit at night when the arch is illuminated. Security screening is airport-style.