Robert Ballard is the finder of important lost things.

In 1985, he discovered the Titanic scattered beneath the Atlantic Ocean. He and his team also located the giant Nazi battleship Bismarck and, more recently, 18 shipwrecks in the Black Sea.

Ballard has always wanted to find the remains of the plane Amelia Earhart was flying when she disappeared in 1937. But he feared the hunt would be yet another in a long line of futile searches.

“You have it in a holding pattern in your head,” says Ballard, founder of the Ocean Exploration Trust. “You’re still saying, ‘No, no, it’s too big a search area.’”

Then, a few years ago, another group of explorers found clues so compelling that Ballard changed his mind. Now, not only is he certain he knows where the plane is, he has set course for a remote atoll in the Pacific island nation of Kiribati to recover it.

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If his expedition succeeds, he’ll not only solve one of the enduring mysteries of the 20th century. The 77-year-old explorer will also be transferring his legacy of discovery to a new generation of oceanic detectives.

Until recently, Ballard accepted the US navy’s version of Earhart’s fate: on 2 July 1937, near the end of their round-the-world flight, the aviator and her navigator, Fred Noonan, vanished over the Pacific. After a lengthy and costly search, the navy concluded, on 18 July 1937, that the two died shortly after crashing into the ocean.

But in 2012, an old friend presented Ballard with a startling alternative.

Kurt M Campbell, who served as assistant secretary of state for east Asian and Pacific affairs in the Obama administration, invited Ballard to a meeting. The two had known each other since their days in naval intelligence.

Campbell ushered him into his office, Ballard recalls: “He closed the door, and he said, ‘I want to show you a picture.’”

First, he offers Ballard a grainy black-and-white photo. “He said, ‘What do you see?’ I said, ‘I see an island with a ship on a reef?’ And he said, ‘No, look over to the left.’”

As Ballard squinted at the blur, Campbell handed him a second, digitally enhanced image. Campbell said the smudge was landing gear from a Lockheed Model 10E Electra. And the reef in the picture was part of tiny Nikumaroro, in the mostly uninhabited Phoenix Islands.

There it was, a precise place to look for Earhart’s plane.

“I went, ‘I’ll be damned,’” Campbell says. “‘That really narrows the search, doesn’t it?’”

The old photograph was taken by Eric Bevington, a British colonial officer, in October 1937, three months after Earhart disappeared. Bevington and his team had scouted Gardner Island, now called Nikumaroro. A British freighter had run aground years before on the northwest corner of the island, and the young officer snapped a picture of it.

Bevington didn’t know he had also captured something sticking out of the water. The Bevington Object, as it became known, was less than 1mm long – a tiny speck near the edge of the frame.

Decades later, an organisation called The International Group for Historic Aircraft Recovery, or Tighar, received Bevington’s pictures. The group is a nonprofit organisation dedicated to aviation archaeology and aircraft preservation. It has been heavily involved in searching for Earhart at Nikumaroro.

Fascination with Earhart’s disappearance has led to wild theories: that she was an American spy captured by the Japanese, or that she lived out her days after assuming a false identity as a New Jersey housewife.

Those who believe in the crash at Nikumaroro say it was along Earhart’s stated navigational line.

The navy even followed clues based on distress calls and dispatched the Colorado, a battleship, from Pearl Harbour in Hawaii, to search the Phoenix Islands. But Ballard and Tighar researchers believe tides would have dragged the plane into deeper waters by the time it arrived at Nikumaroro.

That really narrows the search, doesn’t it? Robert Ballard

According to the official report, a search pilot saw “signs of recent habitation” there. But because nobody waved them down, the search team left and the navy dismissed the theory. What the sailors didn’t know was that the island had been uninhabited for 40 years.

Others say it’s unlikely the island was where Earhart’s life ended.

Dorothy Cochrane, curator for general aviation at the National Air and Space Museum, believes that Earhart crashed in the ocean near Howland Island, Earhart’s original destination, hundreds of miles to the northwest.

In 2010, the notion that the real site may be Nikumaroro got a boost when Jeff Glickman, a forensic imaging expert for Tighar, spotted the blur in the Bevington photo and concluded its shape was consistent with Lockheed Electra landing gear.

Armed with this clue, Richard E Gillespie, the director of Tighar, reached out to Campbell, an avid Earhart fan, for a second opinion.

Nikumaroro, 1,600 miles south of Hawaii (The LIFE Picture Collection via ) (The LIFE Picture Collection)

Campbell shared the photo with experts at the National Geospatial-Intelligence Agency, who used classified technology to enhance the picture. It was sent to intelligence analysts at the Pentagon, who independently concluded the object looked like the landing gear of a Lockheed Model 10E Electra, Campbell said.

So Campbell called Ballard to see if he thought it was a good idea to support Gillespie’s 2012 mission to Nikumaroro, one of a dozen Tighar has made to the island, but the first to search underwater.

That expedition was unsuccessful. But the group didn’t have the funding or capabilities of Ballard and his team. And with his ship, the Nautilus, now in the Pacific Ocean, and its other research obligations completed, Ballard is ready to focus on the search for Earhart.

“The more I read, the more I was convinced I could do it,” he says.

Beyond his 60 years of experience, Ballard’s ship is equipped with a suite of high-definition cameras, a 3D-mapping system and remotely operated underwater vehicles, or ROVs, one of which can descend nearly 20,000ft.

Imagine searching the side of a volcano at night with a flashlight Robert Ballard

But that doesn’t mean the expedition will be easy.

Viewed from above, Nikumaroro is small and flat. But the island is only the plateau of a steep underwater mountain rising 10,000ft from the ocean floor. Earhart landed on the very edge of the island, Ballard believes. As tides rose, her plane may have slipped down the underwater slope.

The ridges of the mountain are rugged – full of troughs and valleys that can hinder sonar. After using onboard technology to create a 3D map of its sides, the team will have to search the mountain visually, monitoring video feeds from the ROVs in 12-hour shifts.

“Imagine searching the side of a volcano at night with a flashlight,” Ballard says.

Gillespie fears what’s left of the Electra might be no more than scattered debris. Still, Ballard’s technology gives him hope. Even those who doubt the Nikumaroro hypothesis think Ballard’s high-tech search at least may prove Earhart was never there.

“It’s time to set that theory straight, and hopefully this will do that,” says Cochrane.

The expedition is being funded by the National Geographic Society, which will record the progress of the Nautilus and its crew for a television programme.

The crew’s efforts will be complemented by a team on the island led by Fredrik Hiebert, the National Geographic Society’s archaeologist-in-residence.

For this expedition, Ballard will share leadership on the Nautilus with Allison Fundis, an up-and-coming explorer he hopes will eventually take his place.

“I feel like Leakey handing it off to Jane Goodall,” he says, referencing her mentor, the paleoanthropologist Louis Leakey.

Ballard feels strongly about promoting women, especially as the Nautilus searches the ocean for one of history’s great female pioneers. Women make up just over half of the crew of the ship.

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Fundis says she is thrilled to be sharing leadership of the Earhart expedition.

“She just had a remarkable life and was a remarkable person, with a sense of bravery that broke down barriers and expectations at a time when society kind of felt like a woman really shouldn’t or couldn’t accomplish what she did,” Fundis says.

The two explorers are confident they will find the Electra.

“Science explorers are like an ideal gas,” Ballard jokes. “They can expand to fill any volume, but they can only do work under pressure.”

Then he laughs, “And the pressure’s on.”