A subterranean museum below architect Eero Saarinen's iconic arch in St Louis has been renovated and expanded by three architecture firms, as part of an effort to rejuvenate the area around the monument.
Formerly known as The Museum of Westward Expansion, the new Gateway Arch Museum was led by New York firms Cooper Robertson and James Carpenter Design Associates, with locally based Trivers Associates.
It encompassed the gut renovation of 105,800-square-foot (9,829-square-metre) existing building, alongside a 47,000-square-foot (4,366 square-metre) addition.

The museum now features a new glazed curved entrance, along with an impressive addition of exhibition space under a grassy knoll.
"The new circular stainless steel and glass entrance refers to the arch in its materiality and form," said Cooper Robertson. The original space was part of Finnish-American architect Eero Saarinen's plan for the site, which opened in 1967. It sits below his monumental, stainless steel arch that soars 630 feet (192 metres) into the sky on the bank of the Mississippi River.

Saarinen designed the Gateway Arch in accordance with the following brief: that the monument should act as a memorial to Thomas Jefferson (who advocated for Western expansion); that it should relate to the preservation of the site of Old St Louis with landscaping; and that the project should incorporate a museum about the migration movement of the American Frontier.

By 1800, the western frontier reached St Louis,...