Ten Strange Sounds Scientists Recorded and Couldn't Identify

In the summer of 1997, underwater microphones operated by NOAA's Pacific Marine Environmental Laboratory picked up an extremely loud, low-frequency sound. The hydrophones were there to listen for undersea volcanoes and earthquakes. This was something else, and it was powerful enough to register on sensors more than 3,000 miles (5,000 km) apart. Researchers nicknamed it the Bloop.

The internet did the rest. Because the sound seemed so enormous, people proposed a giant squid, an unknown sea creature, secret military hardware and, inevitably, a monster out of horror fiction. The nickname and the mystery made it one of the most famous recordings in ocean science.

The real answer came from patience. In the years that followed, NOAA moved hydrophones closer to Antarctica and recorded many icequakes: the sounds of huge icebergs cracking and breaking apart. Their acoustic signatures looked strikingly similar to the Bloop. NOAA now describes the Bloop as consistent with an icequake, and in 2005 its researchers traced the sound to ice cracking and calving off Antarctica. The likely source lay somewhere between the Bransfield Strait and the Ross Sea. No monster required, just a very large piece of ice breaking in a very quiet ocean.

Starting in the 1960s, submarine crews in the waters around Antarctica kept hearing a strange, rhythmic sound. It came in repeating bursts of low pulses that, to human ears, sounded a bit like a duck quacking. The submariners called it the bio-duck. For roughly five decades, it showed up again and again on recordings across the Southern Ocean, especially in the southern winter, and off Western Australia too. No one could say what was making it.

The bio-duck was not a one-off oddity like many sounds in this file. It was common and regular, which made the mystery more frustrating. Scientists suspected an animal, but the obvious candidates had never been caught in the act. Without seeing the source while it made the sound, they could not prove anything.

The break came in February 2013, when researchers attached acoustic recording tags to two Antarctic minke whales in Wilhelmina Bay, off the western Antarctic Peninsula. One tag recorded bio-duck sounds just before the whale began a feeding dive. In a 2014 paper in the journal Biology Letters, the team described this as conclusive evidence that Antarctic minke whales make the sound. The solution matters beyond curiosity: scientists can now use decades of archived recordings to track a whale species that lives in hard-to-reach sea ice.

Beginning in late 2016, U.S. diplomats in Havana reported a range of troubling symptoms, and some described hearing strange, grating noises before they fell ill. The Associated Press later released a recording that personnel said captured the sound: a continuous, high-pitched electronic-seeming whine. Speculation turned to sonic or microwave weapons. The health episodes became known as Havana syndrome.

In early 2019, the biologists Alexander Stubbs and Fernando Montealegre-Z analyzed the AP recording. Its dominant frequency, about 7 kilohertz, and its precisely repeating pulse rate closely matched the calling song of the Indies short-tailed cricket, a Caribbean insect, with echoes added as the sound bounced off indoor surfaces. They concluded the recording most likely captured crickets, not a device, while stressing that their work did not explain what caused anyone's symptoms.

That broader question has drawn separate investigations. In 2024, two National Institutes of Health studies found no MRI-detectable brain injury in the affected group, though NIH stressed the symptoms were real. U.S. intelligence assessments in 2023 and 2025 judged it very unlikely that a foreign adversary was responsible for most cases, though not every agency agreed. The verdict here applies only to the recording itself.

In 2011, residents of Windsor, Ontario, began reporting a low, throbbing rumble. Some said it rattled their windows and dishes and kept them awake. At one point, around 22,000 people had complained. Unlike many hums, this one seemed to have a direction: it pointed across the Detroit River toward Zug Island, a heavily industrialized island on the Michigan side, home to a U.S. Steel mill.

That was a hard lead to follow. Zug Island is private, guarded and closed to outsiders, and Canadian researchers commissioned to study the noise were not allowed to take measurements there. Using microphones placed around Windsor, they concluded the sound most likely came from the island, but they could not prove it. Puzzlingly, the steel mill had operated for about a century, and officials said no new equipment had started up when the noise began.

Then came a natural experiment. U.S. Steel announced in late 2019 that it was winding down operations on Zug Island, and its blast furnaces were shut down by April 2020. Reports of the hum stopped. A researcher involved in the Canadian study said he knew of no evidence that the hum had continued. Exactly which piece of equipment caused it, and why it started in 2011, was never publicly established.

On May 19, 1997, NOAA's hydrophones in the eastern equatorial Pacific recorded a sound that slowly dropped in pitch over about seven minutes, like a machine winding down. Researchers called it Slow Down. Less than two years later, on March 1, 1999, the same network caught another eerie signal, an eerie sound nearly three minutes long that was nicknamed Julia. Both were loud enough to be heard thousands of miles from where they began.

For a while, both joined the list of unexplained ocean noises, and online speculation filled the gap. NOAA's researchers had a more down-to-earth candidate: ice. They traced Slow Down to the waters off the Antarctic Peninsula, an area crowded with islands and shoals, and concluded that it was highly probable it came from a large iceberg running aground. The falling pitch matches a drifting iceberg scraping to a stop on the seafloor.

Julia got a similar explanation. NOAA says its most likely source was a large iceberg grounding off Antarctica, the same kind of event as Slow Down. Because the direction of arrival was uncertain, Julia's origin could lie anywhere between the Bransfield Strait and Cape Adare. Researchers have since recorded similar signals many times as big icebergs ground, which is a strong sign the explanation fits.