In 1997 I travelled to Cherbourg in Normandy with fellow underwater archaeologist Robert Neyland, intent on collecting images of the CSS Alabama - a Confederate privateer sunk off the coast during the American Civil War.

Although bad weather made that impossible, providence smiled, allowing us several days to tour the World War II landing beaches used in the Allied invasion of Nazi-dominated Europe.

The seas were heavy on D-Day

Standing at Point du Hoc - the imposing 100-foots cliffs scaled by American troops on 6 June 1944 - we could not help but wonder what remained of the enormous D-Day invasion fleet beneath the waves below us.

The naval operation mounted on D-Day was without question the most massive in the history of war - but in the 53 years since the invasion no underwater archaeological research had been carried out.

Lost history

Instead, the undersea record of that momentous event had been subjected to decades of erosion, and the clearing of any shipwreck which might prove a hazard to marine navigation.

Faced with the continued loss of this historical record, my Institute of Nautical Archaeology at Texas A&M University, in cooperation with Robert's Naval Historical Centre's Underwater Archaeology Branch, embarked upon the first archaeological reconnaissance of the area in the summer of 2000.

	Duplex Drive Sherman Weight: 35 tons Speed: 25 mph on land, four knots in water Engine: 410 hp diesels - linked to two propellers

Using state-of-the-art remote sensing, detection and imaging equipment, our goal was to determine the location of landing craft, artillery, ships, ordinance and any other equipment lost during the fighting to establish a toehold in occupied Europe.

By mapping the sites we hoped that the shroud of confusion that has surrounded the history of the invasion could be lifted and a detailed history of the losses close to shore could be written.

Soon innumerable magnetometer anomalies - deviations in the earth's magnetic field due to the presence of a massive, ferrous objects - were detected.

Tank find

Perhaps most intriguing was a collection of vehicles assumed to be Sherman tanks, located at a considerable distance offshore at Omaha Beach.

We assumed these were most likely the remnants of a unit of ill-fated amphibious tanks assigned to support the US infantry in the first wave of the invasion.

The DD tanks had to sail 4km alone

These so-called Duplex Drive tanks of the 741st Armoured Battalion were launched from landing craft four kilometres from the beach. Fitted with large canvas skirts round the upper portion of the vehicle, the DD tanks were designed to float low in the water - appearing to the enemy as nothing more menacing than a rubber boat.

The entry of this first group into the rough seas proved disastrous. The tanks were intended to operate in seas with a one-foot swell, yet on D-Day the waves rose six feet.

High waves

The heavy seas swamped 27 DDs, sending them to the sea floor.

As a child I regularly played on a Sherman tank in my local park in Hamilton, Massachusetts - the hometown of legendary tank commander General George Patton.

Later in life I opted for military service to help meet the cost of my college education, joining the tank corps. The chance to return to Normandy to survey the DD tanks resting on the seafloor proved irresistible.

Divers found almost all the lost tanks

In 2001 and 2002, we collected nearly 30 hours of underwater video. There resting on the seafloor we saw the machines, some upright, some on their sides, and several with turrets detached and lying close by.

Once all the tanks were located, studied and filmed - and having interviewed the 10 survivors of the assault - we began piecing together the most accurate history of their tragic part in the invasion.

It seems as if the tanks were sent into the sea 6,000 yards out, as planned, but in a decision which didn't take the conditions into consideration.

Swamped

Furthermore, the landing craft carrying them were drifting away from the target beach - forcing the tanks to set a course which put them side-on to high waves, thus increasing the amount of water splashing over and crumpling their canvas skirts.

Two tanks - skippered by men with enough peacetime sailing experience to know not to turn their sides to the waves - actually made it to the beach.

Tanks played a vital part in the invasion

It had been widely believed the other tanks sunk almost immediately on leaving the landing craft, but our work showed some had struggled to within 1,000 metres of dry land.

In fact some of the sinking tanks had had time to radio following units with a warning not to launch so far out - undoubtedly saving both lives and tanks vital to the battle.

Given the conditions on D-Day and the clumsiness of the tanks it is little wonder that the 741st met the fate it did, but that they launched at all is a testament to the courage and determination of Allied troops in the face of daunting odds and grave danger.

Journeys to the Bottom of the Sea - D-Day: The Untold Story will be screened by BBC Two on 30 May at 21.00 BST