The Ocean Sounds Scientists Still Can't Explain

In 1997, a single sound recorded thousands of miles across the Pacific Ocean sparked countless theories about secret military experiments, underwater volcanoes, and even giant undiscovered sea monsters. It became known simply as the Bloop. Eventually, scientists traced the sound to a massive Antarctic icequake. In fact, scientists have found the origin of many of the most well-known and previously unexplained ocean sounds, such as "Bio-duck" and "biotwang". But not every mysterious ocean sound has reached that point. While decades of research have explained many famous recordings, several still lack a definitive source or scientific consensus. These are the ocean sounds that continue to puzzle researchers.

The Whistle

Unlike many famous underwater sounds that eventually received convincing explanations, the Whistle remains one of the few recorded by the National Oceanic and Atmospheric Administration, or NOAA, that still lacks a confirmed source. The sound was detected on July 7, 1997, by one of NOAA's autonomous hydrophones, underwater microphones originally deployed to monitor low-frequency sounds traveling thousands of miles through the Pacific Ocean. Rather than resembling a whale call or an earthquake, the recording consists of a smooth, undulating tone that rises and falls over several seconds, giving it the whistle-like sound that inspired its nickname.

Scientists believe the most likely explanation is that the sound originated from an underwater volcanic eruption. NOAA notes that the recording closely resembles volcanogenic signals detected from actively erupting submarine volcanoes, including those previously recorded from the Mariana volcanic arc in the western Pacific. Parts of this geologically active region fall within the U.S.-protected Mariana Trench Marine National Monument.

However, the Whistle was detected by only one hydrophone deployed at 08°N 110°W. This is a crucial limitation as researchers need recordings from at least three hydrophones to triangulate a sound's origin. Without that information, NOAA says the source could have been thousands of miles from where it was recorded, making confirmation impossible.

More than 25 years later, the Whistle occupies a rare category in ocean science. It isn't an unexplained sound because scientists have no idea. Instead, it remains mysterious because the leading hypothesis cannot yet be proven. Unlike the Bloop, which NOAA ultimately traced to an Antarctic icequake, the Whistle has never progressed beyond its most likely explanation.

The Upsweep

Most mysterious ocean sounds are heard once and never recorded again. Upsweep has done the opposite. First detected when NOAA's Pacific Marine Environmental Laboratory began operating its autonomous hydrophone network in August 1991, the sound has been recorded repeatedly for more than 30 years. It consists of a series of narrow-band tones that gradually increase in frequency over several seconds and are powerful enough to be detected across much of the Pacific Ocean.

Unlike the Whistle, which was captured only once, Upsweep has become a recurring feature of NOAA's underwater recordings. Despite decades of monitoring, scientists still cannot say exactly what produces it. NOAA has been able to roughly locate the source near 54° south latitude and 140° west longitude, a remote area of the South Pacific close to a region of inferred volcanic seismic activity. That has made underwater volcanic activity the leading explanation. Yet NOAA stops short of calling the mystery solved.

On its official sound archive, the agency states that "the origin of the sound is unresolved." Researchers have also noticed that the sound appears to be seasonal, generally becoming more prominent during the Southern Hemisphere's spring and autumn. It remains unclear whether that pattern reflects changes in the sound's source or simply how efficiently it travels through the ocean at different times of the year.

One thing scientists do know is that Upsweep is changing. NOAA reports that its overall source level has gradually declined since 1991, although the sound can still be detected today by the agency's equatorial autonomous hydrophone arrays. More than three decades after it first appeared, researchers have narrowed the search to one of the most geologically active parts of the Pacific, however, they still cannot definitively explain what is producing it.

The Ping

Unlike NOAA's famous deep-ocean recordings, the Ping was never captured by a global hydrophone network. Instead, it emerged from reports by Inuit hunters and boaters in 2016, who described a persistent underwater pinging or beeping sound in Fury and Hecla Strait, a remote Arctic channel in Nunavut, Canada.

Hunters also reported an unusual absence of bowhead whales, seals, and other marine mammals in an area that had historically been one of the region's most productive hunting grounds, prompting concerns that the mysterious sound was disturbing wildlife. A sailboat later confirmed an unusual acoustic signal using onboard sonar equipment, leading the government of Nunavut to request a formal investigation by Canada's Department of National Defense. However, the investigation only deepened the mystery.

In November 2016, a Royal Canadian Air Force CP-140 Aurora aircraft conducted an acoustic survey of the area under Operation LIMPID, searching for submarines, artificial sound sources, or other underwater activity. According to the Department of National Defense, the crew detected no unusual acoustic anomalies and found no surface or subsurface contacts that could explain the reports. The military ended its investigation without identifying a source, leaving the sound officially unexplained.

Since then, scientists have proposed several possible explanations without reaching a consensus. Ocean acoustics expert Peter Worcester of the Scripps Institution of Oceanography suggested the sound may have been produced by sea ice rubbing, cracking, or shifting within the Arctic strait, a process capable of generating unusual underwater noises. Others speculated about ship sonar, mining surveys, or even submarines, although no evidence has substantiated those theories. Nearly a decade later, no peer-reviewed study or government investigation has definitively identified the source.

Deep Seafloor Arrivals

Not every unexplained ocean sound has a memorable or even spooky nickname. Some are known only by the questions they continue to raise. One of the best examples comes from the 2004 Long-Range Ocean Acoustic Propagation Experiment, or LOAPEX, in the North Pacific Ocean, a major research project led by scientists from institutions including the Woods Hole Oceanographic Institution, the Scripps Institution of Oceanography, and the University of Hawaiʻi. While analyzing low-frequency sound transmissions sent between research instruments separated by up to 1,988 miles (3,200 km), scientists detected a series of unexpected signals arriving at an ocean-bottom seismometer nearly 16,300 feet (4,973 m) below the surface. These signals did not match existing acoustic models and became known simply as deep seafloor arrivals.

The mystery became even stranger when scientists compared the recordings with instruments suspended higher in the water column. The unexplained arrivals were strongest on an ocean-bottom seismometer but were either extremely weak or completely absent on hydrophones positioned about 2,460 feet (750 m) above the seafloor. Existing models of long-range sound propagation successfully predicted the expected arrivals but could not account for this second group of signals. After examining data collected over distances of 31 to 1,988 miles (50 to 3,200 km), the research team concluded that the phenomenon remained unexplained.

More than 15 years later, there is still no scientific consensus on what produced the signals. The leading hypothesis is that they may involve interface waves traveling along the boundary between the seafloor and the sediments beneath it, or previously unrecognized interactions between underwater sound and the ocean floor. Follow-up studies have explored those possibilities, but none has fully explained every observation recorded during LOAPEX. Rather than solving the mystery, the research has highlighted how much scientists still have to learn about the way sound behaves in the deepest parts of the ocean.

Scientists Think They Know the Truth About 'Julia' and 'Slow Down'

Not every mysterious ocean sound remains completely unexplained. In the cases of Julia and Slow Down, scientists have arrived at remarkably similar conclusions. Both recordings were detected by NOAA's Pacific Marine Environmental Laboratory in the late 1990s, and both are believed to have originated near Antarctica. Yet because neither event was directly observed, researchers have never been able to identify the exact iceberg or witness the process that produced either sound.

Julia was recorded on March 1, 1999, by NOAA's autonomous hydrophone network. The low-frequency signal was powerful enough to be detected thousands of miles away, but pinpointing its exact origin proved impossible. After comparing its arrival direction with satellite observations and known iceberg activity, NOAA concluded that the most likely source was a large iceberg running aground off Antarctica. The agency notes, however, that uncertainty in the hydrophone bearings prevented researchers from identifying the precise location or the iceberg responsible.

Slow Down was recorded on May 19, 1997, roughly two years before Julia. Unlike Julia, whose sound remained relatively constant, Slow Down gradually decreased in frequency over about seven minutes, as though whatever produced it were literally slowing down. NOAA considers it highly probable that the sound came from a large iceberg running aground off the Antarctic Peninsula, near 62°S, 60°W. The agency notes that this kind of signal has been recorded many times during iceberg groundings. The sound closely matches the expected physics of a drifting iceberg encountering the seabed, though no one directly observed the specific event that produced this recording.

Together, Julia and Slow Down illustrate an important distinction in ocean science. Researchers don't always struggle because they lack a good explanation. Sometimes they have a compelling one, but the remoteness of the Southern Ocean makes it impossible to gather the direct evidence needed to turn a likely hypothesis into a confirmed fact.

An Ocean Still Full of Unexplained Sounds

The ocean covers more than 70% of Earth, yet scientists have explored only a small fraction of it in meaningful detail. Every year, hydrophones continue recording sounds that span the familiar and the deeply unusual. Some mysteries, like the Bloop, eventually find an explanation. Others, including the Whistle and the Upsweep, remind researchers that even with satellites, autonomous submarines, and some of the world's most sophisticated listening equipment, parts of the ocean remain surprisingly difficult to understand.