World War II Medal of Honor Recipient. He received the award posthumously (presented to his parents) from US Navy Captain G.R. Fairbanks at the Grosse Ile Naval Air Station, Michigan on February 16, 1946 for his actions as a boatswain's mate second class on February 17, 1945, at Pearl Harbor, Hawaii. He enlisted in the US Navy in June 1941 and subsequently served on the battleship USS Idaho (BB-42) and minesweeper USS Advent (AM-83). He then attended the Deep Sea Diving School in Washington, D.C. and was assigned duty with the Salvage Unit under Service Force, Pacific Fleet. On February 17, 1945. at Pearl Harbor, he set aside all personal danger by rescuing one diver who had been trapped in a cave-in of steel wreckage while tunneling under a sunken landing ship, tank (LST). Following this rescue, he went even farther under the buried hulk and, while rescuing a second diver, was pinned down by another cave-in and died as a result. His Medal of Honor citation reads: "For conspicuous gallantry and intrepidity at risk of his life above and beyond the call of duty as a diver engaged in rescue operations at West Loch, Pearl Harbor, 17 February 1945. Aware of the danger when two fellow divers were hopelessly trapped in a cave-in of steel wreckage while tunneling with jet nozzles under an LST sunk in 40 feet of water and 20 feet of mud, Hammerberg unhesitatingly went overboard in a valiant attempt to effect their rescue despite the certain hazard of additional cave-ins and the risk of fouling his lifeline on jagged pieces of steel imbedded in the shifting mud. Washing a passage through the original excavation, he reached the first of the trapped men, freed him from the wreckage, and, working desperately in pitch-black darkness, finally effected his release from fouled lines, thereby enabling him to reach the surface. Wearied but undaunted after several hours of arduous labor, Hammerberg resolved to continue his struggle to wash through the oozing submarine, subterranean mud in a determined effort to save the second diver. Venturing still farther under the buried hulk, he held tenaciously to his purpose, reaching a place immediately above the other man just as another cave-in occurred and a heavy piece of steel pinned him crosswise over his shipmate in a position which protected the man beneath from further injury while placing the full brunt of terrific pressure on himself. Although he succumbed in agony 18 hours after he had gone to the aid of his fellow divers, Hammerberg, by his cool judgment, unfaltering professional skill, and consistent disregard of all personal danger in the face of tremendous odds, had contributed effectively to the saving of his two comrades. His heroic spirit of self-sacrifice throughout enhanced and sustained the highest traditions of the U.S. Naval Service. He gallantly gave his life in the service of his country." The US Navy destroyer escort USS Hammerberg (DE-1015) was named in his honor.