As far as the world knew, Robert Ballard had a singular focus in 1985: to pinpoint the location of the Titanic, the behemoth passenger liner that had disappeared into the depths of the Atlantic Ocean decades earlier, killing more than 1,500 people.

A more complicated truth about the search came into focus years later. Ballard, an accomplished oceanographer, had discovered the ship’s location on the tail end of a top-secret U.S. Navy investigation into the fates of two sunken nuclear submarines.

At the time, the public was meant to be blind to Ballard’s main mission.

“The Navy never expected me to find the Titanic, and so when that happened, they got really nervous because of the publicity,” Ballard told National Geographic in 2008. “But people were so focused on the legend of the Titanic they never connected the dots.”

The 1912 sinking of the ship with 2,200 passengers on board captured the public imagination, prompting hundreds of songs, dozens of books and a handful of movies, as well as new safety measures for ships. Attempt after attempt had been made to find the liner’s wreckage, but the depth of the sea, harsh conditions and conflicting reports of its position when it sank meant that previous efforts had failed.

In 1982, Ballard approached military officials asking them to fund submersible technology to look for the Titanic. An official from the Navy’s submarine warfare program responded that they would provide money for the device but not for finding the infamous passenger liner.

Instead, they wanted Ballard to travel to the sites of the USS Thresher and USS Scorpion, which had sunk in the north Atlantic Ocean in 1963 and 1968, respectively. The Navy wanted Ballard to photograph the wrecks. It was interested in the fate of the submarines’ nuclear reactors and whether there was any evidence that the U.S.S.R., still immersed in the Cold War with the United States, had shot down the Scorpion.

If Ballard finished that mission early, he could look for the Titanic, located somewhere between the downed submarines. But Navy officials doubted he would have enough time to find anything, he said later.

Ballard, who had been a naval intelligence officer for 30 years, told the military that he would “take whatever I can get,” he recalled to the St. Petersburg Times.

So down Ballard went, into the ocean’s depths to the naval submarines. There, he noticed something about how currents affect debris: Heavier items sink faster, leaving a trail of waste.

Ballard realized that knowledge could be the breakthrough he had been looking for. If he could find the Titanic’s line of debris, he could find the vessel.

“So it was kind of an arrow if you just knew which way it was pointing,” ABC News host Diane Sawyer said in a 2008 interview with Ballard.

“And it points directly to the ship,” Ballard replied.

The end of Ballard’s naval mission was fast approaching by the time he began his search for the Titanic. On the day of the discovery, he was lying in his bunk on board the research vessel, reading to keep his mind off the stress, when a cook entered the room. The watch team was looking for Ballard.

When Ballard got to the ship’s control van, his colleagues showed him what their sonar- and camera-equipped robot had detected. One of the Titanic’s boilers was visible through the grainy footage, History reported.

Seventy-three years and many failed search missions after the Titanic sank, the world’s most famous ship had finally been found.

The crew celebrated — then paused upon realizing it was almost 2:20 a.m., the time the vessel had slipped 13,000 feet to the ocean floor. They had a brief service for those lost, Ballard told reporters after returning to an ocean research center in Woods Hole, Mass., on Cape Cod.

The search team members were greeted there as heroes, with about 100 reporters crowding the dock and two TV-network helicopters circling in the air. Coast Guard boats blasted whistles as the research ship approached with Ballard standing on a wing, smiling and giving a thumbs-up.

But his triumphant expression belied the true range of his emotions.

“It was one thing to have won — to have found the ship,” Ballard wrote later. “It was another thing to be there. That was the spooky part.”

Although the Titanic had snapped in two, its bow remained upright. A missing skylight offered a glimpse of the spot where an ornate staircase once stood. The ocean floor was littered with china plates, furniture and an unopened bottle of champagne. Chandeliers still hung from ceilings.

The scene, Ballard said, was that of a haunted house — mostly intact, with scores of leather shoes left as the only signs of those who had perished.