Imagine lowering a microphone into the darkest part of the ocean. There is no sunlight. No visible horizon. No familiar sound of waves breaking on a beach. Only darkness—and a vast underwater world that is constantly making noise.
Then, suddenly, your instrument records something strange. A powerful sound rises from the depths, travels through the water, and appears on recording systems hundreds or even thousands of miles away.
This is not the beginning of a science-fiction movie. Scientists really have recorded extraordinary sounds beneath the ocean. Some were eventually explained. Others remain difficult to identify.
Among the most famous are The Bloop, Upsweep, and a sound once known as Julia. Their stories reveal something fascinating: the ocean can be noisy, mysterious, and surprisingly difficult to understand.
Why Does the Ocean Make So Much Noise?
The deep ocean may look silent from the surface, but underwater it is anything but quiet. Sound is one of the most important ways information travels through the marine environment.
Whales and dolphins use sound to communicate, navigate, find food, and locate other animals. Fish and invertebrates can also produce sounds, while storms, earthquakes, volcanic activity, shifting ice, and moving ships create additional underwater noise.
According to NOAA, sound can travel much farther underwater than light can penetrate, making acoustics an essential tool for studying the ocean.
Scientists use instruments called hydrophones—essentially underwater microphones— to record this hidden soundscape.
The Bloop: The Sound That Started a Mystery
In 1997, researchers monitoring the Pacific Ocean detected an unusually powerful underwater sound. It was so distinctive that scientists gave it a memorable name: The Bloop.
Hydrophones positioned thousands of kilometers apart detected the signal. Because of its enormous apparent strength and unusual character, the sound quickly became the subject of speculation.
Could it have been produced by a gigantic unknown animal? Was it an underwater volcanic event? Or was something else happening in the remote Southern Ocean?
For years, the mystery remained popular among ocean enthusiasts. But science eventually produced a much less mythical—and much more fascinating—answer.
The Bloop Was Not a Sea Monster
NOAA researchers eventually determined that the sound was consistent with icequakes: enormous cracks and fractures produced when large pieces of Antarctic ice break and move.
In other words, one of the most famous "mysterious sounds" ever recorded did not come from an undiscovered creature. It came from the enormous forces acting on Antarctic ice.
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The discovery is a perfect example of how science works. A strange observation can produce extraordinary theories, but researchers continue collecting evidence until a physical explanation fits the data.
Upsweep: A Sound Scientists Still Cannot Fully Explain
But not every strange ocean sound has received such a satisfying explanation.
One of the most intriguing examples is known as Upsweep. NOAA's Pacific Marine Environmental Laboratory reports that the sound was already being detected when its recording system began operating in 1991.
Upsweep consists of a series of narrow-band sounds that rise in frequency. The signal can be detected across a large portion of the Pacific Ocean and appears to have seasonal variations.
Scientists have been able to estimate its general location, near an area associated with volcanic seismic activity. Yet its exact origin remains unresolved.
That distinction is important. "Unexplained" does not mean "supernatural." It simply means that scientists do not yet have enough evidence to identify the source with confidence.
Julia: Another Strange Signal From the Pacific
Another famous sound was recorded on March 1, 1999. It became known as Julia.
NOAA's acoustic researchers later concluded that the most likely explanation was a large iceberg running aground on the seafloor near Antarctica.
The event demonstrates how difficult it can be to determine the origin of a sound recorded thousands of kilometers away. Scientists must compare arrival times, signal characteristics, geographic information, and other environmental evidence.
How Can a Sound Travel So Far Underwater?
One reason these sounds can appear so mysterious is the unusual way sound behaves in seawater.
Water is denser than air, and underwater sound can travel faster and farther than it normally does through air. Temperature, salinity, depth, and frequency all influence how sound propagates.
Under the right conditions, low-frequency sounds can travel across enormous distances. This allows hydrophones to detect events that happen far from the recording instrument.
It also means that an underwater recording can sometimes provide information about events that scientists cannot directly observe.
The Ocean Has Its Own Soundscape
The mysterious recordings are only one part of a much larger underwater soundscape.
Scientists routinely record sounds produced by whales, earthquakes, storms, volcanic activity, ice, fish, ships, and other sources.
Modern acoustic monitoring has become an important scientific tool. NOAA operates networks of hydrophones and other acoustic instruments to study the ocean's natural environment and monitor changes over time.
Could There Still Be Sounds We Cannot Explain?
Absolutely.
The ocean is enormous, and scientists are still learning how its physical processes, animals, and ecosystems interact.
New acoustic recordings can reveal previously unknown biological behavior, geological activity, or environmental changes.
But there is an important lesson behind the famous ocean sounds: mystery should lead to investigation, not immediate conclusions.
The Bloop was once imagined by some people as evidence of a gigantic creature. Eventually, evidence pointed toward Antarctic ice. Upsweep, meanwhile, remains unresolved.
And that may be the most exciting part. The ocean does not need sea monsters to be mysterious. The real planet is already strange enough.
The Mystery Continues
Some underwater sounds have clear explanations. Others remain under investigation. As scientists deploy better hydrophones and explore more of the deep ocean, today's unexplained signals may become tomorrow's discoveries.
Frequently Asked Questions
What was the Bloop sound?
The Bloop was a powerful underwater sound recorded in 1997. NOAA later determined that it was consistent with an icequake caused by large Antarctic ice.
Was the Bloop caused by a giant sea creature?
There is no scientific evidence that the Bloop was produced by a giant sea creature. NOAA's research identified Antarctic icequakes as the most likely source.
What is the Upsweep sound?
Upsweep is a recurring underwater acoustic signal detected across the Pacific. Its general location has been estimated, but NOAA states that its origin remains unresolved.
What is a hydrophone?
A hydrophone is an underwater acoustic sensor used to record sounds beneath the surface. Scientists use hydrophones to study marine animals, geological events, ice activity, and human-made ocean noise.
Why are underwater sounds important?
Sound is essential to many marine animals and is also a powerful scientific tool. Acoustic recordings can reveal animal activity, earthquakes, volcanic events, ice movement, and changes in the underwater environment.
Sources & Further Reading
- NOAA Ocean Service — The Bloop and its scientific explanation.
- NOAA Pacific Marine Environmental Laboratory — Acoustic Monitoring Program.
- NOAA PMEL — Upsweep acoustic recording and analysis.
- NOAA Fisheries — Understanding Sound in the Ocean.
Image rights: Images used in this article are sourced from Wikimedia Commons and NOAA materials identified as Public Domain. Always verify the license on the original source page before downloading, modifying, or republishing an individual file.
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