At 17,000 feet below the surface, the temperature of ocean water hovers just above freezing. It is an eerie black void, too deep for sunlight to penetrate. Sparse oxygen and relatively calm currents help entomb objects that may plunge down from above.

In other words, ideal conditions for finding a preserved airplane, according to marine explorer David Jourdan.

In deep water off a tiny Pacific Island, Jourdan hopes to solve one of aviation’s greatest mysteries – the fate of famed pilot Amelia Earhart.

Jourdan and his Cape Porpoise, Maine-based company, Nauticos, plan to launch an expedition next spring to sweep the ocean floor in a 1,000-square-mile swath of water with sonar.

It is the latest in a string of missions to learn what happened to Earhart when she, her navigator and their Lockheed Electra plane disappeared nearly 70 years ago.

“Things tend to last a time” in the deep ocean, said Jourdan. “Our expectation is the plane will be largely, if not completely, intact.”

That is, if the plane is even in the ocean in the first place.

There is a host of theories about what befell Earhart and navigator Fred Noonan as they made one of the final legs of a widely heralded flight around the world.

Some, like Jourdan, have searched the sea, believing the plane ran out of gas. Others think she survived a crash landing, only to die on a deserted island. The Japanese captured and executed her, according to one theory. The conspiracy-minded claim Earhart survived, living out her life under an assumed name as a New Jersey housewife.

The race to close out the Earhart legend has many elements and characters of a B-movie treasure hunt – eccentric millionaires searching for sunken relics and fame, academic types slashing through jungles in search of artifacts, and others who have devoted their lives and fortunes to solving a decades-old mystery.

They squabble over records of wind speed and the fuel stores of the Electra, pore over British colonial records and nautical charts, and try to divine meaning from old radio transmissions. They are passionate about the hunt and are a competitive bunch.

“These guys that go out searching the deep water may as well pile money on the deck and shovel it overboard,” said Ric Gillespie of Wilmington, Del., who has led several digs on a remote island where he thinks Earhart crash-landed and likely perished. “It’s crazy.”

This much is agreed on – Earhart and Noonan vanished July 2, 1937, as they approached an airstrip on Howland Island. The pair had taken off from Papua, New Guinea, just 7,000 miles short of their goal to make Earhart the first woman to fly around the world.

A fearless flier, Earhart set a string of altitude, distance and endurance records in the 1920s and 1930s, proving the still-young world of flying wasn’t reserved for men.

Tall with a tousle of closely trimmed hair and a wide smile, Earhart captivated a Depression-era America eager for dashing heroes. She lent her name to a line of clothes for “active” women, was feted by presidents and compared to Charles Lindbergh. The press dubbed her “Lady Lindy.”

After Earhart and Noonan disappeared, the Navy launched a rescue operation that stretched for weeks. Ships and planes searched 250,000 square miles of ocean around Howland Island and a nearby chain of small islands. No trace was ever found of the pair or their plane.

That’s because they probably ran out of gas and crashed at sea, according to Elgen Long, a former commercial pilot who has spent the past 30 years researching the Earhart mystery and wrote a book with his wife on the subject five years ago.

Long, 77, of Reno, Nev., will go along on the Nauticos mission as a consultant. He’s been working out at a gym to ready his sea legs.

He believes the answer to Earhart and Noonan’s fate lies in their radio communications with a U.S. Coast Guard cutter that was tracking their course from near Howland Island.

Using logs from the cutter’s radio operator, Long concluded Earhart was perilously low on gas because of a headwind much stronger than she had anticipated. One of her last radio calls said she had only a half hour of fuel left and couldn’t see land.

“We can follow her all the way across the Pacific,” he said of the radio records. “She ran out of gas just when she said she was going to.”

Jourdan has plotted a section of the ocean to the west of Howland Island to search using a sonar system to make images of the ocean floor and any objects that may rest there.

This is his second search of the area – a 2002 mission was aborted partway through because of technical problems. The general area was searched in 1999 by another mission that found nothing conclusive.

Jourdan said his latest search will use better sonar technology and more accurate information on where the plane may have crashed.

The shortage of oxygen and the fairly still waters means it is likely a metal airplane would not have completely corroded over the past 70 years, he said.

Any human remains would have long vanished, but Jourdan hopes to find clues that would show Earhart and Noonan went down with the plane. Jewelry in the pilot’s seat, or perhaps even Earhart’s leather jacket.

“It is certainly possible we will find evidence of her in the seat,” he said. “That would be eerie.”

If found, Nauticos plans to return on another mission to raise the plane. It would be restored and become the centerpiece of a traveling exhibit on Earhart’s life, Jourdan said.

The discovery mission won’t be cheap. Jourdan estimates it will cost $1.5 million to mount. He hoped to auction four slots on board on eBay, with packages starting at $80,000 and going up to $250,000 for a couple, but no bids had been made as of last week.

Earhart’s stepson, George Putnam, was 16 years old when her plane disappeared. Putnam, now 83 and living in Florida, said he supports the mission partly because it could end the wild speculation about what happened to his stepmother. He doesn’t mind if Nauticos salvages the plane either.

“If they can raise it and service it, that would be fine. Let’s see what happens,” he said.

Gillespie calls the mission a “treasure hunt” because of Nauticos’ plans to profit from exhibiting the plane if it’s recovered. But to Long, it could be his last chance to solve one of the 20th century’s biggest mysteries.

“We need the true story of what happened,” he said. “The history we read needs to be correct.”