Deep Sea Detectives The Lost Tanks of D-Day WWII's D-Day Invasion would be history's largest seaborne assault--over 4,000 ships carrying 170,000 troops. A secret weapon was to lead the way: the Duplex Drive (DD) Tank--Shermans modified to "swim" to shore. Scheduled to hit the beach before foot soldiers, without them the infantry would be sitting ducks. And that's what happened at Omaha Beach--28 out of 30 DD Tanks sank before reaching shore. John Chatterton travels to Normandy and joins French diver Bertrand Sciboz in search of sunken tanks.