The well-preserved Manasota Key Offshore site lies in the Gulf of Mexico off Sarasota County about 300m out from shore and 6m deep, and so far the bodies of at least six people have been found, the Florida Department of State has announced.

Much older burial sites exist on land in the USA but scientists are excited by the find because they had never expected to find anything organic preserved in the Gulf of Mexico, which has a sandy seabed.

In this case the bottom of an inland freshwater peat pond had been used to inter the dead in prehistoric times, and it had stayed intact as sea levels rose, surviving millennia of erosion and storms.

Submerged offshore prehistoric burial sites are rarely found, some of the only other examples being in Israel and Denmark.

At first the jawbone find led the authorities to suspect that the site was a modern crime-scene, but forensic investigation indicated that the tooth was that of someone with a prehistoric diet.

The true nature of the 3000sq m site was confirmed through non-invasive survey work carried out by a dive-team led by Underwater Archaeology Supervisor Dr Ryan Duggins of the Bureau of Archaeological Research (BAR), part of Florida’s Division of Historic Resources.

The unusual level of preservation of the site was illustrated by the discovery of wooden stakes that had been sharpened, notched and charred. Wood is normally the first material to decompose but carbon-dating showed them to have been carved 7200 years ago.

The dead would have been wrapped in fibre shrouds, remains of which have also been found, before being sunk in the pond with the fire-tempered stakes marking their position.

The burial sites of the ancestors of Native American peoples such as Florida’s Miccosukee and Seminole are sacred to the tribes and protected under state law, so the survey  work has been carried out in consultation with the tribes, and the site is being monitored and its location kept secret to keep unauthorised divers away.

BAR is developing a long-term management plan focusing on protection and preservation of the site, where it expects to find many more bodies. It also hopes to explore other nearby areas to see if the burial site is part of a larger ancient community.

“Seeing a 7000-year-old site that is so well-preserved in the Gulf of Mexico is awe-inspiring,” said Dr Duggins. “We now know that this type of site exists on the continental shelf - this will forever change the way we approach offshore archaeology.”

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