Just in time for Amelia Earhart's 115th birthday—feted online with a Google doodle Tuesday—comes news from the latest expedition investigating the fate of the aviator, who disappeared during a round-the-world flight in 1937.
The news isn't especially promising—Earhart's plane remains missing. But the so-called Niku VII expedition pursued only one of three main Earhart-disappearance theories, leaving fertile ground for future searches.
The team, funded by the International Group for Historic Aircraft Recovery (TIGHAR), is currently on its way back to Hawaii after a five-day sonar and video search around Nikumaroro.
TIGHAR said in an online statement that it was disappointed that it had not made a "dramatic and conclusive discovery," but that evidence for the plane might still be hidden in their data.
"Did TIGHAR's Niku VII expedition find the Earhart aircraft? It's far too early to say," reads the latest daily report posted online by the Delaware-based group.
"Big pieces of airplane wreckage were not immediately apparent, but after 75 years in Nikumaroro's severe and unstable underwater environment, that is hardly surprising. Whatever survives is hard to find."
Watch Full Nat Geo Documentary: Where's Amelia Earhart?
NGC explores one of the most fiercely debated mysteries of all time: What really happened to Amelia Earhart?
TIGHAR's $2.2-million expedition was originally slated to last ten days, but "equipment problems directly attributable to the severity of the underwater environment at Nikumaroro" cut the search time to only five days.
"In that time we saw no objects that we recognized as aircraft debris, but we have volumes of sonar data and many hours of high-definition video to review before we'll know the results of this expedition definitively," TIGHAR's statement said.
Internet-marketing executive Justin Long of Vancouver is partly funding a Simon Fraser University project to collect Earhart's DNA from envelopes for potential matching against any human remains that might be discovered.
"Given the area that they've plotted and the area that she was last spotted and heard from ... [it's not unexpected that they didn't] find anything on the first search, because it really is like looking for a needle in a haystack," said Long, who was not involved in the expedition.
Previous Earhart expeditions to other areas have lasted months, not days, he noted.
TIGHAR president Pat Thrasher told the Los Angeles Times that the group is already planning another voyage for next year.
Amelia Earhart: What We Know
Earhart's fate remains one of aviation's greatest unsolved mysteries.
What is known is that Earhart and Noonan were flying Earhart's Lockheed Electra on a 2,556-mile (4,113-kilometer) route from New Guinea (map) to Howland Island (map) on the third-to-last leg of their equatorial circumnavigation.
The pair was scheduled to land on tiny Howland on July 2, 1937. From there, they were to go on to Hawaii and California.
The U.S. Coast Guard cutter Itasca was at Howland to assist Earhart in this pre-radar era by providing radio bearings and a smoke plume, but owing to radio problems, communication was sporadic and broken.
According to the Itasca's radio logs, Earhart indicated she must be near the island but couldn't see it and was running low on gas. The Electra never made it to the island.
At its height, the ensuing search effort involved more than 3,000 people, ten ships, and at least 65 planes, to no avail. The official U.S. position is that Earhart and Noonan ran out of fuel and crashed in the Pacific Ocean.
(Read Amelia Earhart's speech on accepting the National Geographic Society's Special Medal.)
Earhart Theory 1: Nikumaroro Castaway
According to TIGHAR executive director Richard Gillespie, after Earhart reported being low on fuel, she began searching for Howland. Her path erred toward the southwest, since she knew the Phoenix Islands, an alternative landing site, lay 350 miles (560 kilometers) in that direction, he suggests.
TIGHAR's investigation into this theory has uncovered native accounts of a pre-1939 plane wreck on Nikumaroro and reports of two castaways, a man and woman who fit descriptions of Earhart and Noonan.
TIGHAR has been to the island on several occasions to collect evidence. In 2003, for example, the team went to investigate potential wreckage that a marine biologist had spotted on a coral reef, but a storm washed away the object before TIGHAR arrived.
"But in looking in the village, we recovered three more artifacts that prove to be of the same type of airplane part we had recovered on our first trip there in 1989: a dado"—an interior kickplate for protecting wiring, Gillespie told National Geographic News in 2003.
Dados are common in civilian aircraft but not military planes, suggesting that the part could have come from Earhart's Electra.
Unfortunately, said Gillespie, dados are considered furnishings of the airplane cabin and are not included in Lockheed's blueprint drawings of the Electra. And none of the extant Electras in museum collections have their original dados, he added.
In 2010 TIGHAR discovered a bone on Nikumaroro that may have been part of a human finger. But researchers at the University of Oklahoma later said they could not determine whether the fragment had belonged to the freckle-faced aviator.
TIGHAR is also attempting to locate 13 supposedly human bones that were discovered on the island in 1940 but that disappeared after being sent to Fiji.
Earlier this year a TIGHAR report revealed that the researchers had found on Nikumaroro fragments of 1930s-era U.S. beauty and skin care containers—including one for a freckle concealer—that may have belonged to Earhart. A full report is expected to be published in October.
Finally, before their July 2012 mission was cut short, TIGHAR was scanning the seas around Nikumaroro to investigate the theory that a photograph taken by a British Colonial Service officer three months after Earhart's disappearance had inadvertently captured components of her plane's landing gear.
(Also see "Expedition Scours Pacific for Amelia Earhart Wreck" .)
Earhart Theory 2: Open-Ocean Crash Near Destination
About ten years ago Nauticos—a Hanover, Maryland, company that performs deep-ocean searches and other ocean-research services—led an effort to locate Earhart's plane where they believe it crashed: in the Pacific Ocean in the vicinity of Howland Island.
David Jourdan, Nauticos' president, said in 2003 that, by studying factors such as Earhart's broken-up radio transmissions and what is known about the Electra's fuel supply, he and his colleagues had narrowed down an area of the ocean that they believe will eventually yield the plane's grave.
"We are confident it is in the area we are searching," said Jourdan. "Of course, we cannot guarantee it, because it could be on the outside edge, but we are sure it is in the vicinity."
In March and April of 2002, the company used a high-tech, deep-sea sonar system to search 630 square miles (1,630 square kilometers) of the ocean floor near Howland. They didn't find the plane on that expedition or a 2006 follow-up mission.
Later, in 2009, a team organized by the Waitt Institute for Discovery searched a roughly Delaware-size area just west of Howland with the help of deep-sea robots. Though the expedition turned up no clues, an optimistic Ted Waitt, the institute's president, said in a statement, "our results eliminate thousands of square miles from future search efforts." (Waitt collaborates on a series of grants with the National Geographic Society, which owns National Goegraphic News.)
Earhart Theory 3: Conspiracy
A third theory is that Earhart and Noonan, unable—or perhaps not intending—to find Howland, headed north to the Japanese-controlled Marshall Islands (map), where they were taken hostage by the Japanese, possibly as U.S. spies.
Some believe both pilots were eventually killed, while others believe Earhart and maybe Noonan returned to the U.S. under assumed names. According to one theory, Earhart took the name Irene Craigmile, then married Guy Bolam and became Irene Bolam, who died in New Jersey in 1982.
"If she couldn't find Howland, Plan B was to cut off communications and head for the Marshall Islands and ditch her airplane there," Rollin C. Reineck, a retired U.S. Air Force colonel who lives in Kailua, Hawaii, claimed in 2003.
Reineck authored a book, Amelia Earhart Survived, in which he describes Earhart ditching her plane in the Marshall Islands and returning to the U.S. under an assumed name for national security reasons.
According to Reineck, the scheme would have allowed the U.S. government to rescue Earhart in the Marshall Islands and at the same time perform prewar reconnaissance on the Japanese.
"However, the plan went bad, as a lot of plans do," said Reineck. Earhart radioed that she was headed north, the message was intercepted, and the Japanese took her hostage, he claims.
Reineck argues the mission was kept secret because the U.S. public wouldn't have tolerated the government putting the beloved Earhart in harm's way.
According to Reineck, expert analysis has determined that several photographs of Irene Bolam, her handwriting, and other forensic evidence indicate her connection to Amelia Earhart.
Reineck's interpretation, though, isn't exactly widely supported. But until Earhart's wreckage is hauled from the Pacific, the mystery surrounding her disappearance will continue to invite speculation of every stripe.
As California-based naturalist and Amelia Earhart enthusiast Ronald Reuther told National Geographic News in 2003, "There are still U.S. governmental documents concerning Earhart and her disappearance that are still held secret by our government. Why?"