Operation Thunderhead Operation Thunderhead was a highly classified combat mission conducted by U . S . Navy SEAL Team One and Underwater Demolition Team 11 ( UDT - 11 ) in 1972 . The mission was conducted off the coast of North Vietnam during the Vietnam War to rescue two U . S . airmen said to be escaping from a prisoner of war prison in Hanoi . The prisoners , including Air Force Colonel John A . Dramesi were planning to steal a boat and travel down the Red River to the Gulf of Tonkin . Lieutenant Melvin Spence Dry was killed on the mission . He was the last SEAL lost during the Vietnam War . His father , retired Navy Captain Melvin H . Dry , spent the rest of his life trying to learn the circumstances surrounding his son ' s death . The details , however , were long shrouded in secrecy . Mission . In April 1972 , SEAL Team One left Subic Bay in the amphibious - transport submarine . The plan was to launch the team at night from the submerged submarine in a Swimmer Delivery Vehicle ( SDV ) piloted by two UDT - 11 operators and head for a small island off the mouth of the Red River . On 3 June 1972 , Lieutenant Dry decided to conduct a clandestine reconnaissance mission that night . Shortly after midnight the team launched from " Grayback " but a combination of navigation errors and strong current took them off course . After an hour , the crew was compelled to abort the mission . They were unable to locate " Grayback " and they were forced to scuttle their underpowered SDV after its battery power was exhausted . The next morning , the team was rescued a few miles off the coast . The four men were flown to the , Operation Thunderhead ' s command ship . At 2300 hours on 5 June , the men were to be transported back to " Grayback " by helicopter . The team would perform a night water drop next to the submarine . During briefings with the pilots , Lt . Dry and Chief Warrant Officer ( CWO ) Martin emphasized the maximum limits for the drop were " 20 / 20 " — 20 feet of altitude at an airspeed of 20 knots , or an equivalent combination . When the helicopter arrived near " Grayback " ' s expected position , they could not locate the submarine . As they desperately searched for the submarine ' s beacon , Lt . Dry and his men prepared to enter the water and lock into the submerged submarine . According to CWO Martin , the drop was conducted downwind , adding another 15 to 20 knots of forward velocity when the jumpers hit the water . Lt . Dry died immediately of " severe trauma to the neck " caused by impact with the water , according to the Navy ' s death report . Two other team members were badly shaken , and one was seriously injured . Several hours earlier , " Grayback " launched its second SDV but the team abandoned their mission when their air ran out ; subsequently , they made an emergency free ascent to the surface . After seeing a strobe light and hearing voices , the two teams rendezvoused . At about 0100 , they found Lt . Dry ' s lifeless body , inflated his life vest , and held him in tow as they swam seaward to be rescued . An HC - 7 helicopter rescued the men at dawn and returned them to the " Long Beach " . On 12 June , the remaining team members were transferred back to " Grayback " . With the likelihood of a successful prisoner escape by sea lessened by the recent U . S . mining of North Vietnam ' s ports and rivers , Operation Thunderhead was soon terminated . Because the mission was classified , the Navy did not acknowledge Dry ' s death as a combat loss until February 2008 , when he was posthumously awarded the Bronze Star for Valor .