It was one of the bloodiest days in World War II history that most people have never heard of.
In the weeks leading up to the D-Day invasion, Allied forces conducted a training exercise off the southern coast of England to simulate the landing of large forces on enemy soil. German torpedo boats got wind of the exercise and sank two troop vessels, killing more than 700 soldiers and sailors.
Now an underwater robot from the Massachusetts company whose autonomous submarines have surveyed the wreck of the Titanic and searched for Amelia Earhart’s plane has captured vivid images of the remains of the lost vessels.
In March, a Remus 100 submarine made by Hydroid Inc. of Pocasset used sonar to find and capture images of the landing craft lost during Exercise Tiger, a simulation of the Allied invasion of Normandy that would take place about a month later. Hydroid carried out the survey at its own expense, to commemorate the 70th anniversary of the sinking.
Underwater robots are in wide use around the world for deep-water surveying, naval surveillance, and oceanographic research. A similar robot made by Bluefin Robotics Inc. of Quincy is being used to search the southern Indian Ocean for traces of missing Malaysia Airlines Flight 370. That sub, Bluefin-21, can dive to 15,000 feet, far deeper than the Remus 100 used in the search for the World War II wrecks.
But a larger model from Hydroid, the Remus 6000, can dive to 20,000 feet. In 2011, three of those were used to find the remains of Air France 447, which crashed in the Atlantic Ocean in 2009.
The quest to photograph the two WWII troop carriers was sparked by Graham Lester, a British citizen and a Hydroid vice president. During a vacation last summer, he encountered a seaside memorial in Torcross, England, to those who died in Exercise Tiger.
“He’s scratching his head and going, how come nobody knows about this?” said Hydroid’s Kathy Forrester.
Indeed, Exercise Tiger remained a secret for weeks after the sinking, because the Allied command did not want to give away vital intelligence about its invasion plans.
The two sunken ships were among a group of eight carrying about 4,000 American support troops, who were to follow up the first wave of the invasion. On April 28, 1944, they had sailed about 30 miles off the southern coast of England, and were to make their practice landing in an area called Slapton Sands, because it resembles one of the Normandy invasion areas called Utah Beach.
But the German navy picked up radio traffic from the American ships, and ordered in nine torpedo boats. During the ensuing battle, three of the eight landing craft were hit by torpedoes. While one made it back to England, two sank.
Incredibly, the carnage continued as the remaining landing craft reached the beach. British artillery had been ordered to shell the landing zone, to make the training more realistic. But they were supposed to cease fire once the American landing began. They didn’t, and over 200 more Americans died from friendly fire.
In all, Exercise Tiger took the lives of nearly 1,000 men; the actual landing at Utah Beach resulted in fewer than 200 dead.
After visiting the Exercise Tiger memorial, Lester came up with the idea to create images of the lost landing craft. He got the backing of his superiors at Hydroid, which is owned by Kongsberg Maritime of Norway, and the company donated its time and equipment for the search.
Britain’s Royal Navy also offered to help, and conducted a separate survey of the area.
Tracking down the lost ships was relatively easy, as the general area of the sinkings was well known. The wrecks lie in about 150 feet of water, within the Remus 100’s relatively shallow diving range of about 300 feet.
The robot carries high-resolution sonars on either side of its hull, and can sweep the ocean bottom in a back-and-forth, lawn-mower-like pattern to find sunken objects.
Mike Mulrooney, a Hydroid senior field service technician, said that once the general location of the ships was pinned down, the Remus 100 could pass fairly close to the wrecks. Hydroid also increased the speed at which its sonar “pinged” the target with sound waves, resulting in sharp, detailed images of the ships.
“The faster we can ping, the more data points are made across the body of the wreck,” said Mulrooney.
The images produced by the Hydroid team will be donated to the UK National Archives and to local organizations that maintain memorials to Exercise Tiger. A Hydroid spokeswoman said there are no plans for further exploration of the site.
Under US and international law, the wreckage of warships sunk in battle remain the property of the nations for which they fought, and are to be treated as grave sites. Such wrecks may not be salvaged without the permission of the nation that owns them.Hiawatha Bray can be reached at email@example.com. Follow him on Twitter @GlobeTechLab.