English: The most prominent feature of the site is the Hortonsphere. The Hortonsphere was built as the containment building for the SIR prototype (Submarine Intermediate Reactor) a sodium cooled design that would eventually power the USS SEAWOLF. At the same time, a prototype was being built in Idaho for the STR prototype (Submarine Thermal Reactor) a pressurized water reactor that would power the USS NAUTILUS. The Hortonsphere (or the D1G Ball as it will eventually become known as) was the first pressure containment building ever constructed and, at the time, was the largest sphere ever built. It was built by the Chicago Bridge and Iron Company under a contract from The Rust Engineering Company in the early to mid 1950s. Photo courtesy of Former West Milton, NY Historian Karen Staulters.