Built in 1929 as the construction facility for the U.S. Navy's rigid airships USS Akron (1931) and USS Macon (1933), the Airdock was a colossus of engineering acumen and structural steel. These airships, or dirigibles, served as the fleet's aerial watchdogs, but the Navy no longer needed them with the advancement of aircraft carriers, though they were large enough to carry five biplanes. Eleven steel parabolic arches, cresting at 211 feet, create one of the largest open-space interiors in the world and shelter more than 364,000 square feet of floor space. Only one arch is fixed to its concrete piling, and its 660-ton spherical doors rest on flatbed railroad cars to open. The Airdock is a National Civil Engineering Landmark and was added to the National Register of Historic Places in 1973.