IT TOOK 73 years to find the wreckage of the Titanic at the bottom of the sea — but it might never have been discovered if it wasn’t for a curious navy chief on a completely different mission.

A new exhibit, Titanic: The Untold story, at the National Geographic Museum in Washington has revealed the once top-secret story about oceanographer Robert Ballard’s discovery of the wreck while searching for two nuclear submarines.

The navy commander, who found the wreck on the first day of September 1985 after only 12 days of searching, was tasked to explore sunken submarines the USS Thresher and the USS Scorpion using submersible technology.

The submarines sank in the North Atlantic Ocean during the Cold War, and the US government wanted to learn the environmental impact on the subs and find out whether foul play was involved.

But Ballard wanted more from the mission and asked for funding to locate the Titanic after several expeditions to find the wreckage had previously failed, largely due to the difficulty of reaching a wreck that lies nearly 4km below the surface, where the water pressure is over 3000kg per square inch.

But Ballard believed its remains were near the submarines and wanted a chance to find it.

Others were given months to locate the ship. Ballard and his team had less than two weeks after completing their first mission.

He studied every detail of the Titanic and decided to seek not the ship itself, but the debris field. He (correctly) theorised it had broken in half and left a debris trail as it sank.

At 2am on September 1, they found the RMS Titanic wreckage at the bottom of the Atlantic Ocean, off the coast of Newfoundland, about 2000km from New York.

Ballard and several crew members watched the robotic submersible technology deliver images of the Titanic’s boiler, which hadn’t been sighted since it was above water all those years ago.

“We were at the very spot Titanic sank. We were there,” Ballard told National Geographic.

The Navy didn’t expect Ballard would find the Titanic, so when that happened, “they got really nervous because of the publicity,” he said, which is why the story is only now being told.

The story is being told in detail at the National Geographic exhibit, which is now open to the public through January 6, 2019.