Signals From Nowhere: 10 Radio Signals That Defy Explanation

Since the early 20th century, radio waves have opened up a new way of seeing the universe, offering insight into everything from distant galaxies to deep-space weather. But amid the hum of data and routine transmissions, some signals have refused to play by the rules. Strange bursts, repeating pulses, and eerie one-offs have puzzled scientists for decades. Some turn out to be quirks of human technology, others the byproduct of celestial phenomena. And yet, a handful still linger in the grey area between known science and the unknown, sparking curiosity and, occasionally, conspiracy.

From deep-space enigmas to the signal known only as WOW!, here are ten radio transmissions that continue to baffle the experts.

At their core, radio signals are a form of invisible energy that travel through space as waves. They move at the speed of light and can cross vast distances, passing through dust, gas, and even whole galaxies. On Earth, radio signals carry familiar things like music, phone calls, and Wi-Fi. In space, they allow scientists to study distant objects.

Radio signals come from both natural and human-made sources. Some are steady, while others are brief and unpredictable. To understand them, astronomers look at three key features: strength, timing, and frequency.

One important category is bandwidth. Narrowband signals are squeezed into a very small range of frequencies. This makes them stand out from natural background noise and is why they often attract special interest. In contrast, broadband signals spread their energy across many frequencies and are more common in natural sources like stars.

Signals are also grouped by behaviour:

• Continuous signals change slowly over time.
• Pulsed signals arrive in regular beats, like those from pulsars. Pulsars are ultra-dense, dead stars that spin rapidly and send out radio pulses in precise, regular beats, often many times each second.
• Burst signals are short, powerful flashes that may never repeat.
• Artificial signals come from human technology, such as satellites and radar.

Occasionally, however, a signal appears that doesn’t fit comfortably into any category. One such signal, detected in 1977, would become one of astronomy’s greatest mysteries.

Few radio signals have achieved the legendary status of the “Wow! Signal.” It was detected on 15 August 1977 by astronomer Jerry R. Ehman while he was analysing data from a large radio telescope at Ohio State University. Among hours of routine cosmic noise, one signal stood out sharply from the rest.

What made it so striking was its narrowband nature. As explained earlier, most natural radio sources spread their energy across many frequencies. This signal, however, was tightly focused into a very small frequency range, making it unusually clear and hard to dismiss as random interference. It also appeared to come from the direction of the constellation Sagittarius.

Ehman was so surprised that he circled the signal on a computer printout and wrote a single word beside it: “Wow!” The name stuck.

The signal lasted just 72 seconds and was never heard again, despite repeated attempts to find it. Scientists have suggested possible explanations, including passing space objects or rare natural effects, but none fully account for its properties. Today, the Wow! Signal remains one of the most intriguing unanswered questions in modern astronomy.

Fast Radio Bursts are short, powerful flashes of radio waves that last just a few milliseconds. They were first noticed in 2007 and have since been found in galaxies across the universe. Because they are so brief and bright, FRBs release vast amounts of energy in an instant.

For many years, scientists debated what could produce them. One leading idea has been magnetars, neutron stars with incredibly strong magnetic fields. In 2025, astronomers at MIT used a novel method to trace one burst, known as FRB 20221022A, to the turbulent magnetic region around an ultradense neutron star. This was done by analysing how the signal “twinkled” as it travelled through space, similar to how stars appear to twinkle in Earth’s sky.

This discovery provides the first clear evidence that at least some FRBs come from extremely compact objects, though many questions remain. A small number of FRBs repeat irregularly, while others appear only once, and the full range of their causes continues to be a topic of active research.

Among all known fast radio bursts, FRB 121102 holds a special place. Discovered in 2012, it was the first FRB observed to repeat, instantly ruling out many catastrophic explanations, such as supernovae, which only happen once.

This repeating signal originates from a dwarf galaxy roughly three billion light-years from Earth and is associated with a persistent radio source, possibly a nebula or an accreting black hole. Its bursts show complex frequency patterns and extreme polarisation, hinting at an unusually powerful magnetic environment.

While magnetars remain the leading explanation, the exact mechanism behind FRB 121102’s repeated outbursts remains uncertain. Its behaviour continues to stretch existing models of astrophysics.

In 1997, underwater microphones operated by the US National Oceanic and Atmospheric Administration picked up an ultra-low-frequency sound in the Pacific Ocean. Dubbed “the Bloop,” it was louder than any known marine animal and was detected by sensors thousands of miles apart.

Initially, the signal sparked speculation about enormous, unknown sea creatures lurking in the depths. Later analysis suggested it was most likely caused by an icequake – the fracturing of a massive iceberg.

Yet some scientists remain cautious. The sheer intensity and unusual frequency profile of the sound set it apart from typical ice-related noises, ensuring that the Bloop retains a place in the annals of unusual signals, hovering somewhere between the mystery and the mundane.

The centre of the Milky Way is a crowded and energetic place. A supermassive black hole sits there, surrounded by dense star clusters, powerful magnetic fields, and clouds of gas and dust. Because visible light is blocked by this dust, astronomers often turn to radio waves to study what’s happening in the heart of our galaxy. These waves can pass through the fog and carry unique information about hidden processes.

Among the known radio emissions from this region are a handful of Galactic Centre Radio Transients (GCRTs). These are signals that switch on and off without a clear explanation. One especially puzzling example is ASKAP J173608.2−321635, nicknamed Andy’s Object after its discoverer. Detected several times between 2020 and 2021 using the Australian Square Kilometre Array Pathfinder (ASKAP) and MeerKAT telescopes, this source appears and disappears at irregular intervals, sometimes varying dramatically in brightness. It shows an unusually high degree of polarisation (a trait that means the radio waves are aligned in a particular direction) and no counterpart has been found at optical, infrared, or X-ray wavelengths.

Astronomers have ruled out many ordinary explanations, such as flaring stars or typical pulsars, but no single model yet explains all of the signal’s behaviour. Its discovery suggests there may be new classes of radio-emitting objects near the galactic centre that remain to be fully understood.

In the early 2000s, astronomers studying galaxy clusters observed a faint radio signal emanating from the Perseus Cluster, one of the most massive structures in the nearby universe. The emission appeared to form bubble-like structures around the cluster’s central galaxy, Perseus A.

These radio bubbles are thought to be linked to activity from a supermassive black hole, inflating cavities in surrounding hot gas. However, the precise mechanics of how these signals are generated and sustained remain unclear.

Long-period radio transients, or LPTs, look similar to pulsars at first glance, but behave very differently. Instead of pulsing many times a second, they produce brief radio flashes only every few minutes or even hours. Each burst can last seconds or minutes, followed by long periods of silence.

This slow rhythm is puzzling. Standard theory suggests objects spinning this slowly should stop producing strong radio signals. Yet some LPTs remain active for years and show highly polarised emissions, hinting at unusual physics. Scientists suspect either rare, extremely magnetised neutron stars or white-dwarf systems. For now, LPRTs remain one of radio astronomy’s strangest discoveries.

The Sun is a surprisingly prolific source of strange radio emissions. Among the most enigmatic are “zebra pattern” bursts. These are signals which appear as multiple, parallel stripes across radio frequencies during intense solar flares.

These patterns have been observed for decades, yet their precise origin remains debated. They likely involve complex interactions between plasma waves and magnetic fields in the Sun’s atmosphere, but no single model has fully accounted for all observed features.

Understanding these signals is more than an academic exercise. Solar radio emissions can affect satellites, communications, and power grids on Earth, making their mysteries directly relevant to modern life.

In 2020, astronomers using powerful radio telescopes discovered enormous, faint circular structures in the sky, now known as Odd Radio Circles. These vast rings, visible only in radio wavelengths, span millions of light-years and have no obvious counterparts in visible light, X-rays, or infrared observations.

Several hypotheses have emerged, including shockwaves from ancient cosmic explosions or interactions between galaxies and intergalactic gas. Yet none fully explain their size, symmetry, and rarity.

When Jocelyn Bell Burnell first detected a regular radio pulse in 1967, its astonishing precision led to a tongue-in-cheek label: LGM-1, short for “Little Green Men.” The signal pulsed every 1.3 seconds with remarkable consistency, unlike anything previously observed.

Further study revealed it to be the first known pulsar, a rapidly rotating neutron star emitting beams of radiation. While pulsars are now well understood, the initial confusion surrounding LGM-1 serves as a reminder of how easily natural phenomena can masquerade as something far more exotic. Even today, newly discovered pulsars sometimes display behaviours that challenge existing theories, keeping the legacy of LGM-1 alive.

Radio astronomy has transformed the understanding of the universe, revealing phenomena that are invisible to conventional telescopes. Yet for every mystery solved, new questions seem to constantly pop up, carried across space on waves of unseen energy. These ten signals serve as excellent reminders that, somewhere in the static, there may yet be discoveries waiting to be heard.​