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The term deep sea diving refers to underwater diving, usually with surface supplied equipment, and often refers specifically to the use of standard diving dress. Traditional the helmets were made of copper. Our Naval Mark V Deep Sea Diving Helmet Keychain is made of 100% real brass. This keychain is 100% brass that comes on a swivel brass chain with a screw lock carabineer. This makes a perfect gift for the mariner in your life.