The first JIM suit was completed in November 1971 and underwent trials aboard in early 1972. In 1976, the JIM suit set a record for the longest working dive below , lasting five hours and 59 minutes at a depth of . The first JIM suits were constructed from cast magnesium for its high strength-to-weight ratio and weighed approximately 1,100 pounds (498.95 kg) in air including the diver. They were 6 ft 6 inches (1.98 m) in height and had a maximum operating depth of 1,500 feet (457 m). The suit had a positive buoyancy of . Ballast was attached to the suit's front and could be jettisoned from within, allowing the operator to ascend to the surface at approximately 100 feet (30 m) per minute. The suit also incorporated a communication link and a jettisonable umbilical connection. The original JIM suit had eight annular oil-supported universal joints, one in each shoulder and lower arm, and one at each hip and knee. The JIM operator received air through an oral/nasal mask that attached to a lung-powered scrubber that had a life support duration of approximately 72 hours. Operations in arctic conditions with water temperatures of -1.7°C for over 5 hours were successfully carried out using woolen thermal protection and neoprene boots. In 30°C water the suit was reported to be uncomfortably hot during heavy work.