At 17,000 feet beneath the surface, the temperature of ocean water is just above freezing, oxygen is sparse and currents are relatively calm.

In other words, ideal conditions for preserving an airplane that might have crashed into the depths nearly 70 years ago, according to marine explorer David Jourdan, who hopes to answer one of aviation’s greatest mysteries: the fate of famed pilot Amelia Earhart.

Jourdan and his Maine-based company, Nauticos, plan to launch an expedition in the spring using sonar to sweep a 1,000-square-mile swath of ocean bottom west of tiny Howland Island in the Pacific Ocean.

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 on a flight around the world.

“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.

There is a host of theories about what befell Earhart and navigator Fred Noonan in 1937 as they made one of the final legs of their widely heralded flight.

Some have searched the sea, believing the plane ran out of gas. Others think she survived a crash landing but died on a deserted island. Another theory is that the Japanese captured and executed her. The conspiracy-minded claim Earhart survived and lived out her life under an assumed name as a New Jersey housewife.

This much is agreed on: Earhart and Noonan vanished July 2, 1937, as they approached an air strip on Howland Island, roughly midway between Australia and Hawaii. They 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.

Famed aviator Amelia Earhart and her navigator, Fred Noonan, pose in front of their twin-engine Lockheed Electra in Los Angeles in May 1937, before their attempt to fly around the world. Marine explorer David Jourdan plans to launch an expedition in spring 2005 to sweep the ocean floor in a 1,000-square-mile swath of water with sonar to find Earhart's missing plane.

The Navy launched a weeks-long search of 250,000 square miles of ocean around Howland and a nearby chain of small islands. No trace was found of the plane.

One of those going along on the Nauticos mission is Elgen Long, a former commercial pilot who has spent 30 years researching the mystery.

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.”