While echoes are nothing out of the ordinary in radio transmissions, they usually occur about 1/7 of a second after the original transmission, the amount of time it takes a radio wave to travel once around the Earth.

In 1927, Jørgen Hals spoke to ghosts—or so it seemed. The Norwegian engineer was picking up radio signals from a Dutch shortwave transmitting station when he began to notice strange echoes of the original transmission.

This theory was later expounded upon by science fiction author Duncan Lunan in 1973, who wrote about a 13,000 year old alien probe from the constellation Boötes hiding out in the vicinity of the moon and echoing back our messages.

One of the more outlandish explanations of these echoes invokes an alien civilization trying to communicate with us. In 1960 Ronald Bracewell proposed in Ronald Bracewell proposed in Nature that if we were to be contacted by an autonomous artificially intelligent alien probe, the messages we received would most likely sound like the echoes reported by Hals and Strømer in 1928, the reflection of our own radio signals back to us being a highly energy efficient mode of establishing contact.

Hals' original observation of what are now known as Long-Delayed Echoes (LDEs) set off a frenzied investigation into this unsettling phenomenon in the following years, although just what accounts for these echoes still remains a mystery.

Hals didn't even attempt to explain what the signal meant. "From where this echo comes I cannot say for the present," Hals wrote to Strømer shortly after his discovery. "I can only confirm that I really heard it."

The bizarre thing about these echoes, Hals wrote in a letter to the Norwegian physicist Carl Strømer , was that they were occurring up to three seconds after the original transmission and demonstrated a severe declension in amplitude that wouldn't be expected if the signal had merely made several trips around the Earth.

"The planet reflection hypothesis is literally quite far-fetched. Communications to our nearest neighbors Venus and Mars takes from five minutes and up round-trip, so with delay times in seconds or tens of seconds they can be dismissed," said Holm.

In the former case, there is a chance that the signals are being reflected back from the moon or some of the other planets, which might account for the staggering variety in the delays. In 1946 the first signal intentionally bounced off the moon took approximately 2.5 seconds, as did signals between Houston to the Apollo 11 crew in 1969. More recently, amateur experiments successfully bounced signals off of Venus , which resulted in a delay of about five minutes.

A number of more plausible explanations have been proposed in the years since Hals's initial discovery, mostly notably by A.G. Shlionskiy in a 1989 article for Telecommunications and Radio Engineering . Shlionskiy proposed two basic mechanisms: signals reflected from outer space, and signals reflected terrestrially.

Although scientists have yet to settle on a final explanation for these mysterious echoes, Holm believes this is has less to do with a lack of scientific knowledge than a lack of willpower

"The starship hypothesis is a very interesting one, and the one which seems to be the most popular one on the internet," said Sverre Holm, a professor of signal processing at the University of Oslo. "Such theories always excite our imagination, but it builds on a very poor data set. Unfortunately I believe it says more about human imagination than anything else."

According to Holm, the most likely cosmic culprit accounting for LDEs is the collection of ionized gas clouds in Earth's Lagrange regions, which would account for echoes of between two and 10 seconds.

"Clouds of plasma are taken seriously as a possible explanation, however one would expect frequency shifts due to relative movement as well as massive attenuation in this case, and that doesn't seem to be the case," he said.

The most likely explanations for LDEs, however, are terrestrial and relatively mundane. The leading explanation attributes the echoes to a process called "ducting," wherein radio signals are guided through the Earth's ionosphere. When the radio wave reaches the other side of the Earth, it is reflected off the upper ionosphere and travels back along the same path, accounting for the delay.

"I am definitely on the side of the Earth-bound explanations," said Holm. "Especially the theory involving mode conversion in the ionosphere's plasma. It involves conversion from a radio wave to an acoustics-like wave, a plasma oscillation, and back again."

Unfortunately for Holm's favored mode conversion theory, the effects are very difficult to study outside of a laboratory and no one has been able to explain the conditions under which it occurs in the Earth's ionosphere.

Although scientists have yet to settle on a final explanation for these mysterious echoes, Holm believes this is has less to do with a lack of scientific knowledge than a lack of willpower.

"I think that with today's satellites and sensors, the mystery of LDEs could probably be solved," he said. "What's holding us back is most likely the problem is not considered important enough—it doesn't occur often enough and doesn't affect important enough forms of communications."

Perhaps the problem will only be considered important enough when the 13,000 year old aliens hiding behind the moon come to attack—until then, the mystery of the ghosts in the radio waves lives on.