In 1935, a Navy airship sank in Monterey Bay; when researchers found the wreck decades later, rockfish and thornyheads were already using it as habitat

There is something notable about a wreck that eventually becomes a home for marine life. The Monterey Bay National Marine Sanctuary's records describe the USS Macon as a 785-foot Navy airship that sank tail first off Point Sur on 12 February 1935. Its exact resting place remained unknown for nearly 50 years. Then a local fisherman caught a piece of the airship's girder in his net and displayed it on a wall in a Moss Landing restaurant. The fragment was recognised by Marie Wiley Ross, whose father had commanded the Macon and survived its sinking. The discovery helped confirm that wreckage had been found in the area and became an important clue in the search. Later investigations eventually led researchers to the wreck site, which was located during dives by the Monterey Bay Aquarium Research Institute and the US Navy in 1990 and 1991, more than 1,000 feet below the surface.The Macon was no ordinary airship. It was one of two rigid airships built by the Goodyear-Zeppelin Corporation, the other being its sister ship, the USS Akron. Unlike the soft-bodied blimps familiar today, it had a rigid skeleton made from duraluminum, with three internal keels, catwalks and girders forming the structure inside. The airship had space for about 100 officers and crew to live and work, and it could carry its own aircraft. Four Curtiss Sparrowhawk biplanes could be launched and recovered while the Macon was airborne through a hangar opening in its belly. Pilots used a trapeze system to hook their aircraft back onto the airship, a manoeuvre that earned them the nickname "the men on the flying trapeze."The Macon and Akron also used helium for lift rather than hydrogen. Helium reduced the risk of fire and explosion associated with hydrogen, but it did not protect the airship from structural damage. On 12 February 1935, the Macon encountered a severe storm off the California coast. Damage to the airship's structure caused it to break apart and fall into the Pacific. Two crew members died, while most of those aboard were rescued.Why a sunken airship became habitatWhen researchers returned to the site in 2006 aboard MBARI's research vessel Western Flyer, they used the remotely operated vehicle Tiburon to examine the wreck in greater detail. The expedition documented more than the remains of a historic aircraft. Cameras recorded flattened control cabins, chart tables and other objects that had remained on the seafloor for decades, including a pencil lying among the wreckage.The biological survey also showed that the Macon had become part of the deep-sea environment. Splitnose rockfish and thornyheads were among the species found around the wreck, using the remaining girders and other hard surfaces as habitat. On a soft, sandy seabed, the Macon's metal framework provided solid structures that marine animals could settle around and use for shelter.Researchers mapped two main debris fields. One contained the airship's hangar bay, its four Sparrowhawk aircraft and several engines. The other held the bow section and mooring assembly. Both areas had developed communities of fish and invertebrates by the time scientists surveyed them. The transformation was not planned. The Macon had been built as a naval aircraft carrier in the sky, but after sinking, parts of its structure became habitat on the seafloor.The 2006 expedition also allowed scientists to examine how the wreck's aluminium structure had changed over time. By comparing observations from the 2006 survey with records and images from the 1990 and 1991 dives, researchers could study how the deep-sea environment was affecting the airship's metal. Having observations from the same wreck separated by about 15 years made the site useful not only for maritime archaeology but also for understanding the long-term deterioration of historic material underwater.A wreck worth protectingDocumentation of the Macon continued after the 2006 expedition. In August 2015, the National Oceanic and Atmospheric Administration's Office of National Marine Sanctuaries worked with the US Naval History and Heritage Command and the Ocean Exploration Trust, along with the OceanGate Foundation, to document the wreck again. This survey combined photography and video with 3D modelling, creating a more detailed record of the site.The wreck had already received formal historic recognition. In February 2010, on the 75th anniversary of the disaster, the USS Macon was added to the National Register of Historic Places. NOAA archaeologist Bruce Terrell has described it as the only archaeologically documented rigid airship wreck in the United States and possibly the world.The Macon is important not only because of its unusual engineering or its place in naval history. Its wreck also shows what can happen to a structure after it disappears beneath the sea. An airship that once carried people and aircraft through the sky now rests in two debris fields more than 1,000 feet underwater. Decades after the disaster, its remaining framework had become part of the seafloor, providing hard surfaces and shelter for marine life such as splitnose rockfish and thornyheads. The wreck is therefore both a record of a 1935 disaster and an example of how historic material can eventually become part of a living deep-sea habitat.