The US dive-centre owner found what appeared to be a vast submerged forest of cypress trees, complete with bark, growing around an ancient riverbed.

The site, in the largely barren Mobile Bay, has since been described as the equivalent of a coral reef in the tropics, with anemones growing on the tree stumps and attracting turtles, octopuses, eels, crabs, sharks and abundant fish of many species.

Now researchers using radiocarbon-dating techniques on timber samples recovered by divers from the site have been amazed to find that the trees date from an Ice Age some 60,000 years ago.

The unique underwater forest lying 10 miles off the Alabama coast has been described as a “prehistoric time-capsule” by the scientists from Louisiana State University and the University of Southern Mississippi.

They believe that it can provide new insights into climate development and the forms of life that existed in prehistoric times.

At a time when the land was widely covered by ice-sheets and sea-levels were far lower than today's, the forest would have been set in a valley many miles inland. It is likely that the trees were buried suddenly by a catastrophic event such as flooding, and preserved by being encased in oxygen-light swamp-type sediment.

When tree samples were brought up by divers, the researchers noted that they still contained fresh-smelling resinous sap.

The submerged forest was probably partially exposed in 2004, because it lay directly in the path of the devastating Hurricane Ivan. Analysis of the sediment in which it lies revealed a metre of sand, with a metre of sandy clay below that, and beyond that peat.

The discovery and continuing scientific investigation has now been documented in a half-hour film called The Underwater Forest.

It was directed by Ben Raines, an environmental journalist who has pursued the story from the first reports in 2011 and has carried out many dives at the site. He is hoping to have the area declared a National Marine Sanctuary.

Raines' film can be seen here

Divernet - The Biggest Online Resource for Scuba Divers