The Treste II Sphere was in active service from 1966 to 1973 and was used on over 30 occasions at depths varying from 800 to 16,400 feet. It was also used to obtain photographs of the USS Thresher (SSN 593) at approximately 8500 feet. Fabricated in Oct 1965 and built by NAVSHIPYD Mare Island, the steel alloy HY-120 sphere had an outer diameter of 91.4 inches, a thickness of 3.9 inches, and three viewing windows, each 5.84 inches thick, made of Cady AcrylicPlastic (mil-c-2447). Its depth dive of 16,400 feet was achieved three times, including twice in Nov 1971 and again in Apr 1972.