During a summer afternoon in July 1937, 15-year-old Betty Klenck was sitting on the floor of her home in St. Petersburg, Florida, when something occurred that would stay with her the rest of her life.

The teenager, who loved listening to music on the family's shortwave radio, often kept a notebook handy to record lyrics to her favorite songs and doodle caricatures of the famous voices that permeated the airwaves. Her father indulged her pastime by erecting a powerful antenna on a pole outside the house. As a result, she could often pick up stations from all around the world, she revealed in an interview decades later.

At around 3 p.m., Klenck was cruising the stations for music when she paused on the clearly distressed voice of a woman. The words "This is Amelia Earhart!" were repeated several times, followed by broken transmissions of two people arguing, cries for help and "Water’s knee deep!"

Amelia Earhart and her second navigator Ed Noonan (far left) standing in front of her Lockheed L10 Electra. Only days later, Earhart and Noonan would disappear over the South Pacific. Amelia Earhart and her navigator, Fred Noonan (right), standing in front of her Lockheed L10 Electra. Only days later, Earhart and Noonan would disappear over the South Pacific. (Photo: Public Domain)

Days earlier, Earhart and her co-pilot Fred Noonan had made international headlines after their disappearance during an attempt to circumnavigate the globe. Could what Klenck was hearing possibly be their calls for help from more than 6,000 miles away in the South Pacific?

The signals faded in and out over the next three hours, with Klenck, now spellbound that she was possibly hearing the SOS calls of the famed aviator, recording each one in her notebook. Despite her father relaying the information to Coast Guard officials involved in the search for Earhart and Noonan, the voices heard by Klenck that day were dismissed as nothing more than a hoax.

According to a new study conducted by The International Group for Historic Aircraft Recovery (TIGHAR), the radio signals received by government agencies and so-called "accidental witnesses" like Klenck listening on shortwave radios from Texas to Toronto, are part of 57 transmissions they now say could have credibly come from Earhart and Noonan.

Part of a transmission received by Thelma Lovelace in the Canadian province of New Brunswick at 1:30 a.m. July 7. Part of a transmission received by Thelma Lovelace in the Canadian province of New Brunswick at 1:30 a.m. on July 7. (Photo: Creative Commons/MNN)

"Betty’s notebook describes a scene so clearly authentic and so emotionally powerful that her experience tends to overshadow the other 56 credible signals heard in the days following the Electra’s failure to arrive at Howland Island," TIGHAR writes. "However, in truth, those receptions constitute a body of evidence far stronger than Betty’s alone."

From hoax to grim truth

Why were the signals analyzed by TIGHAR dismissed by search officials over 80 years ago? The simple reason is that officials searching for Earhart adamantly concluded her Lockheed L10 Electra had crashed in the water and subsequently sank. As a result, any transmissions after her last official communication of 8:46 a.m. on July 2 were labelled as "all probably criminally false."

TIGHAR, however, believes that Earhart and Noonan did manage to make an emergency landing — a theory that, oddly enough, is backed up by the curious timing of the pair's distress signals.

"These active versus silent periods and the fact that the message changes on July 5 and starts being worried about water and then is consistently worried about water after that — there's a story there," Ric Gillespie, the group's director, told the Washington Post. "We're feeding it to the public in bite-sized chunks. I'm hoping that people will smack their foreheads like I did."

At the mercy of the tides

TIGHAR believes Earhart and Noonan made a crash landing on the reef surrounding Gardner Island and near the wreck of the SS Norwich City (left). TIGHAR believes Earhart and Noonan made a crash landing on the reef surrounding Gardner Island and near the wreck of the SS Norwich City (left). (Photo: Google Maps)

When Earhart and Noonan ran into trouble sometime on the morning of July 2, a long-standing theory is that the pair crashed near the uninhabited coral atoll of Gardner (also called Nikumaroro) Island. Instead of landing on the island, which is covered in thick scrub and forest, TIGHAR believes the Electra landed at low tide on the relatively smooth surface of the surrounding reef.

Under this scenario, Earhart and Noonan managed to set down the Electra in one piece but faced a new challenge: the rise and fall of the tides. At low tide, when the reef surrounding Gardner runs from dry to less than six inches of water, the propeller of the aircraft could be engaged to charge the battery used to power the radio. Because the engine is air-cooled, however, it would need to be shut down periodically to prevent overheating. At high tide, the prop would not be able to clear the water and charging the batteries would cease.

The breakthrough to support this hypothesis came when the group matched tide data collected on the reef with the times of the collected post-loss transmissions from Earhart and Noonan.

"The correlations were astounding," the group wrote. "Conditions at Gardner were not considered in determining which signals are credible, and yet — night after night — the credible transmissions occurred only when the water level was low enough for the prop to clear."

In addition, the group also found "active" periods and "silent" periods at low tide to support the heating and cooling theory regarding the Electra's engines.

Increasing desperation

Amelia Earhart standing in front of the Lockheed Electra that she and Fred Noonan disappeared in on July 2, 1937. Earhart standing in front of the Lockheed Electra that she and Noonan flew. (Photo: STAFF/AFP/Getty Images)

In meticulous detail, TIGHAR's report overlays the tide data with the individual credible transmissions received in the days after Earhart and Noonan's disappearance.

On the evening of the first day, July 2, Mabel Larremore in Amarillo, Texas, heard Earhart say, "Plane down on an uncharted island. Small, uninhabited." She added that the plane was partially on land and partially in water and that Fred Noonan was "seriously injured." Hours later, Nina Paxton in Ashland, Kentucky, heard the broken transmission of a woman declaring Earhart's radio call sign of KHAQQ followed by "down in ocean," "on or near little island at a point near..." and "our plane about out of gas. Water all around. Very dark."

In the days that followed, everyone from Coast Guard operators on Howland Island, roughly 350 nautical miles north of Garland Island, to powerful commercial stations in Hawaii to even a 16-year-old boy in Wyoming picked up signals later deemed credible by TIGHAR. The reason why Earhart's transmissions were able to carry so far was likely due to the harmonics produced by her radio.

"High harmonic frequencies 'skip' off the ionosphere and can carry great distances, but clear reception is unpredictable," the paper says.

Based on the transcripts, the transmissions leading up to the final credible account paint a picture of rising water, a deteriorating Fred Noonan, and increasing desperation. The last, on July 7 at 1:30 a.m. was picked up by Thelma Lovelace of St. Johns, New Brunswick, Canada.

"Can you read me? Can you read me? This is Amelia Earhart. This is Amelia Earhart. Please come in," Lovelace recorded in a book. "... we have taken in water, my navigator is badly hurt; we are in need of medical care and must have help; we can't hold on much longer."

After the silence

According to TIGHAR's report, the high tide that occurred on July 7 was the largest yet and very well may have either submerged the transmitter or carried the Electra over the reef and into deeper water. What we know for sure if that no credible transmissions exist beyond this date. A flyover of Gardner Island on July 9 by search planes noted high, pounding surf and no evidence of the Electra or her crew.

"Earhart somehow made it to shore and survived for a time as a castaway," the paper concludes. "Noonan's fate is unknown."

In 1940, artifacts such as a sextant box and shoe parts were discovered near skeletal remains on Gardner Island. An analysis of the data collected on the bones (since lost) and conducted decades later by forensic anthropologists found the morphology of the bones to be "consistent with a female of Earhart's height and ethnic origin." A paper published in the journal Forensic Anthropology earlier this year further stated, "Until definitive evidence is presented that the remains are not those of Amelia Earhart, the most convincing argument is that they are hers."

We may never know exactly what happened to Earhart and Noonan in the hours and days that followed their disappearance, but for the TIGHAR research team, the clues left behind point to a heartbreaking ending in the South Pacific.

"For people who believe in science, and pay attention to the evidence," Gillespie told Today. "I'm comfortable that we have an answer that stands up to scrutiny."

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.

Amelia Earhart's final SOS calls detail her haunting last days
Radio signals from aviator Amelia Earhart, received by dozens all around the world, tell a desperate tale of survival.