In the 1940s, the U.S. Navy's combat swimmers wore metal helmets, with long hoses supplying air from the ships on the water's surface.A 24-year-old medical student at the University of Pennsylvania had a better idea. Using technology used for anesthesia, and tested during Family underwater excursions to the New Jersey shore, Christian Lambertsen invented the Lambertsen Amphibious Respiratory Unit, a self-contained breathing system that recycled an operator's air.The device had obvious military applications, especially at the dawn of World War II, allowing Soldiers to conduct long underwater missions without releasing bubbles, which would identify their position.To prove his invention's worth, he met with leaders from the United States' Office of Strategic Services -- the precursor to the Central Intelligence Agency and U.S. Special Operations Command -- at a Washington, D.C. hotel and hopped in the pool, staying underwater until the OSS was convinced that this technology was something they could use."When World War II broke out, everyone realized they needed people to sneak in and blow up ships," said Maj. Trevor Hill, the commander of the U.S. Army Special Forces Underwater Operations School in Key West, Fla. "Everyone -- the French, the Italians, the British, the Americans -- started trying to figure out how to breath underwater, and Dr. Lambertsen was the guy in America who cracked the code."The OSS took Lambertsen's design and created a maritime unit to conduct underwater operations. While Lambertsen was still attending medical school, he began training the OSS's operational swimmers and continued to refine his device.It's a great story on its own, but Lambertsen's story had only begun.Two years later, in 1943, Lambertsen graduated first in the class and accepted a direct commission as an officer in the U.S. Army's Medical Corps; for the duration of World War II, he was assigned to the OSS's Maritime Unit.Lambertsen, a physically fit young man, was not brought in by the OSS to shy away from war-fighting. Eager to serve his nation, he joined the operational swimmers, trained them and other military dive units, and led them on covert underwater missions to attach explosives to Japanese ships.In other words: using the re-breather technology that he invented, Lambertsen swam into enemy-controlled waters to blow up their boats."Someone once described the ideal OSS candidate as a Ph. D. who could win a bar-fight," said Charles Pinck, the director of the non-profit OSS Society. "I think Dr. Lambertsen fits that profile perfectly, because he was out there on missions, which was harrowing business. They were able to do a lot of damage to the Japanese."Lambertsen used this time, and the operational experience, to improve the LARU and research oxygen toxicity, carbon dioxide intoxication, and a motorized underwater canoe for long-range covert infiltration."[The swimmers] clearly had a great impact on the organization's ability to conduct covert operations, there's no question about that," Pinck said. "Aside from [Maj. Gen. William J. Donovan], who founded the OSS, I would place Dr. Lambertsen in the next pantheon of people who were significant in the organization."The OSS was disbanded in 1945, and Lambertsen completed his military service in an Army hospital. He left active duty as a major in 1946, and returned to the University of Pennsylvania's School of Medicine as a member of its faculty. He eventually renamed the LARU as the Self-Contained Underwater Breathing Apparatus, or SCUBA.Over the following 50 years, Lambertsen achieved wide success as an environmental medicine pioneer. He converted an abandoned altitude chamber into a positive-pressure thermal laboratory, and founded the University of Pennsylvania's Institute of Environmental Medicine, where research focused on the realms of the deep ocean and deep space, the healing power and lethality of oxygen, and man's adaptability to high and low pressures. As America ventured into space, Lambertsen made sure the nation's astronauts could stay alive. He served as chairman of the Life Systems Advisory Board for the Mercury and Gemini projects, and on a NASA advisory committee for the International Space Station.His relationship with the U.S. military also lived on.When Maj. Trevor Hill, quoted above, was a cadet at the United States Military Academy at West Point in 1996, he attended the Combat Diver Qualification Course in Key West."Someone put a dive log up on the bulletin board, and it said 'C. Lambertsen -- 15 minutes of bottom time,'" Hill said. "I remember wondering if that was the Lambertsen who invented SCUBA."It was; a group of dive instructors interested in military history had traced the lineage of military combat diving back to Lambertsen's OSS service in World War II."As the school's instructors researched, they realized a rich Army history in diving," Hill said. "The conventional wisdom was that this capability evolved from the Navy, and the Army adapted from them, but it really wasn't."On the school's invitation, Lambertsen traveled down to the school and, in his mid-80s, took a modern-day LAR-V rebreather out for a test-dive. Between that visit and his death in February 2011, Lambertsen made several return trips to speak with the Special Forces dive community's instructors and students."He was inducted into the Special Forces regiment, and we've awarded him his own green beret, Special Forces tab and combat diver badge," Hill said.Lambertsen made his final return to the Special Forces Underwater Operations School on March 10, 2012 -- 13 months after his death -- when his four sons spread his ashes into the Atlantic Ocean, as per his own wishes."My father is known as the father of military combat diving," said David Lambertsen, one of his sons. "We knew we wanted this ceremony to include more than our immediate Family, and we've always considered the military to be an important part of our father's life.""The ceremony reinforced the links that Green Berets share with our forefathers of the OSS," Hill said. "The waters of Key West are ideal for combat diver training, and they were perfect for Maj. Lambertsen's final dive mission."Watched over by Army Soldiers from the U.S. Army John F. Kennedy Special Warfare Center and School, and Navy personnel from Naval Air Station Key West, Dr. Christian Lambertsen was laid to rest in a formal military ceremony about a mile off the coast of Key West.Lambertsen's military legacy lives on; in modern operations, combat divers are an important addition to a battlefield commander's team."From a Special Forces perspective, 90 percent of humanity lives within an hour's drive of a coastline or river," Hill said. "That's where people live, and that's where wars happen. Having the ability to sneak into an area undetected allows us to do the Special Forces mission."For the special-operations students attending Hill's SFUWO, that Special Forces mission is the end-state; combat diving is simply how they may make their commute. Special-operations forces are specifically designed to shape foreign political and military environments to prevent war; in order to prevent wars in hard-to-reach or well-defended areas, Special Forces Soldiers need a way in -- and out."Coming in from the water is one of the only infiltration techniques that has an exit right there; you can get right back in the water and leave if you have to," Hill said. "When you parachute into an area, that's a one-way trip until a helicopter can come get you."For Hill, and his special-operations combat diving instructors, Lambertsen's permanent home in the waters of Key West are an addition to the presence he's already established."Our closed-circuit diving equipment room is filled with his creations," Hill said. "Our hyperbaric recompression chamber is operated using the procedures he developed for many dive injuries, which he discovered. Our submarine lock-out chamber simulates a technique he discovered, developed and practiced during World War II."Lambertsen will go down in history for many significant contributions toward the advancement of science. His 70-year career enabled man to explore air, land, sea and space, Hill said. As a scientist, inventor and environmentalist, his research will live on in academia.At the Special Forces Underwater Operations School in Key West, Lambertsen's legacy will mean a little more to its students and instructors. Under clear skies in 80-degree weather, the ashes of the father of military diving were spread into the choppy waters where modern combat divers learn their trade. To them, Lambertsen's OSS service encapsulates the innovation, bravery and professionalism they strive for each day.