Just before the 70th anniversary of the D-Day invasion on June 6, PBS' "Nova" will debut the new documentary "D-Day's Sunken Secrets" on May 28, following an expedition to locate the wrecks of the ships that were so instrumental in the battle to better understand what happened and why.

"There's three things we're doing in the show," says writer, producer and director Doug Hamilton. "We're talking about the history of the day, how it unfolded. We're talking about the engineering story and what it took to prepare the troops for whatever their particular action was, whether that be the landing crafts or these floating tanks or the gliders that went in, so you get a bit of that back story. Maybe a third of the show is that. Then a third of the show is the expedition. What 'Nova' really wanted to do in this was not just tell the history of what happened, but to participate in a way of seeing this battle more actively and to be a part of adding value to our understanding of what happened on D‑Day, and hopefully that's what the show will do. But it's a mix of chronology as well as what we find on this expedition."

"We follow a team surveying the seabed on the Normandy coast using divers, submersibles, and advanced sonar. What they see are the remains of hundreds of ships that were transporting men and equipment to support the invasion," continues Paula Apsell, series senior executive producer for "Nova." "It's a chance to explore the feats of engineering and logistics that made this invasion possible and paved the way for World War II to end. And it is a last opportunity to bring elderly veterans back to search for their lost ships and pay tribute to those who fought and fell beside them."

One of those veterans is Bill Allen, who served as a medic in WWII and whose ship, LST (523), sank off Omaha Beach on its fourth crossing of the English Channel. As part of the expedition, he descended in a mini-sub to see the wreckage of his ship. "I had never been in a submarine. It was very tight. The diameter of the sub must have been six or seven feet at the most. But I wanted to see my ship, and I was willing to pay the price. When we came up, we had been down an hour and 15 minutes. But it was very exciting. It had a great meaning to me. I had no fear because I was interested in seeing what was left of my ship," one of about 400 wrecks that have been located so far.

"We surveyed 500 square kilometers, which is a 10‑kilometer‑wide area by 50, and we got 400 targets, and we started off with basically 50 to 70 targets," says Sylvain Pascaud, an underwater cinematographer who served as director of operations for the D‑Day expedition. "From a technical standpoint, finding and mapping things underwater is still a very difficult endeavor," he emphasizes. "It's probably easier to find an iPhone in China than to know exactly where a wreck is. Just creating this map is not something automatic. It's something we had to be very creative about. It's a combination of high-tech and low-tech, and it took us 30 days."

Information supplied by fisherman, the British Royal Navy, and the French Navy helped in building that map and identifying wrecks. "Then we tried to match our map with the past, and that's how we found a 40 percent difference and how the wrecks basically were collapsing on themselves and disappearing in the sand," says Pascaud.

"A lot of these D‑Day wrecks are decaying so much faster," adds Hamilton. "In 50 years, they're going to be gone. So it's not only surprising what they're finding, but it's surprising that this incredible history is disappearing and we haven't really explored it that carefully."

The documentary also touches on the role weather, tides, ocean currents and topography played in the D-Day invasion. "They were working against the tides, and the currents in this region can be very dangerous, and if you try to fight everything, you won't succeed," Pascaud points out. "When you look at what went on in the planning, one of the first steps was to understand the geography there," Hamilton continues. "It's a very unique part of the world. The English Channel has these incredible tidal ranges — 25 feet twice a day. Understanding the role that geography played in this invasion is an important part of the story."

These and other factors nearly sunk the operation — no pun intended. “I make the argument that we should have lost,” says Adrian Lewis, professor of history at the University of Kansas, retired U.S. Army major, and author of "Omaha Beach: A Flawed Victory." "It's almost a miracle that we didn't lose Omaha Beach, and if you take a look at German reports coming in, they thought they were winning, and they were."

"I think they knew the casualties were going to be extremely heavy," Allen says of the Allied forces. "Ships were lost. Planes were lost. Many lives were lost. That's the reason we kept taking troops and supplies in, to replace those that had been lost, and it was just a continual thing, bringing in more reinforcements, and that was what won it."

While movies like "Saving Private Ryan" "focus on the incredible heroism at that time," Hamilton says that the documentary offers a different perspective on D-Day. "Seeing all the wrecks that are out there gives us an understanding of how it actually transpired. A lot of the information that exists is wrong," he notes. "Sylvain and his team are correcting the historical record, and we’ve been able to show that."

'Nova' explores D-Day from the ocean's depths
Just before the 70th anniversary of the D-Day invasion on June 6, PBS' "Nova" will debut the new documentary "D-Day's Sunken Secrets."