Beneath the waves, sound travels faster and farther than in air, making it the ocean’s primary language. Whales sing long, complex songs that can carry across entire ocean basins. Dolphins click and whistle in rapid-fire exchanges. Fish croak, drum, squeak and hum. Even tiny crustaceans—snapping shrimp—produce crackling noises so loud that they can mask submarine propellers.

Yet, not all underwater sounds can be explained by known marine life. Over the decades, researchers have recorded noises so strange and so powerful that they have inspired both scientific investigation and wild speculation.

Why the ocean speaks

Sounds in the water are usually connected with the exchange of information. Whales use sound to navigate on their vast migrations. Fish sometimes use it to locate their spawning grounds. Each coral reef has an acoustic signature, which is known to the fish who live there. This signature helps to guide them, including attracting travellers back to their birthplace. 

Scientists studying the soundscape of the ocean therefore discover a variety of sources, including the movements of animals, their migrations, the restlessness and quaking of icebergs and the noises of undersea volcanic activity.

The Bloop, the Julia and the Upsweep

In 1997, hydrophones picked up an incredibly loud, low-frequency sound in the South Pacific, nicknamed the “Bloop”. NOAA described it as rising rapidly in frequency for approximately one minute. It was loud enough to have been picked up on multiple sensors, over a range of more than 5,000km. This audio profile resembled that of a living creature, but it did not resemble any known animal, and it was many times louder than any animal sound yet recorded, the loudest of which is the blue whale.

Eventually, NOAA Vents Program attributed Bloop to a large icequake. Icequakes resemble the Bloop's sonic profile and display a similar amplitude. The Bloop appeared to have originated from icebergs disintegrating near South Georgia Island in 2008, either between Bransfield Strait and the Ross Sea, or near Cape Adare in Antarctica, a well-known source of cryogenic signals. 

“Julia”, recorded in 1999, was long held to be mysterious, but now NOAA believes that the source was most likely a large iceberg that had run aground off Antarctica. It was so loud that it was heard across the entire Equatorial Pacific Ocean, and lasted for two minutes and 43 seconds. 

“Upsweep” was first detected in 1991 when NOAA's equatorial sound surveillance system first began recording. It consists of a long train of narrow-band upsweeping sounds of several seconds in duration and is heard throughout the Pacific.

Upsweep reaches peaks in spring and fall, suggesting that it is seasonal, but the reason for this seasonality remains unclear. The source has been located as being roughly between New Zealand and South America and is speculated to be underground volcanic activity. 

Songs we still do not understand

Some sounds defy clear categorisation. The “bio-duck”, a strange quacking noise recorded in the Southern Ocean, was a mystery for decades until it was finally linked to minke whales in 2014. But other noises continue to escape identification. Scientists believe that the sources must be volcanic and glacial activity. There are recordings of some of these sounds on the Wikipedia page describing them.

The SOFAR channel

The SOFAR channel is short for “sound fixing and ranging channel”, and is sometimes known as the deep sound channel (DSC). It is a horizontal layer of water in the ocean where the speed of sound is at its slowest. Thus, it acts to guide sound waves. Low-frequency sound waves may travel thousands of miles in this channel before dissipating. 

Listening to the future

Today, networks of underwater microphones—some military, some scientific—are constantly recording. As artificial intelligence improves, so does the ability of researchers to sift through these immense sound archives. Each strange click, moan or boom could be a clue to life in the deep, or to changes in the ocean environment that we have yet to fully understand.

The ocean is speaking all the time. And we are now learning to listen.

Ethologist Ila France Porcher, author of The Shark Sessions and The True Nature of Sharks, conducted a seven-year study of a four-species reef shark community in Tahiti and has also studied sharks in Florida with shark-encounter pioneer Jim Abernethy. Her observations, the first of their kind, have yielded valuable details about the reproductive cycles, social biology, population structure, daily behaviour patterns, roaming tendencies and cognitive abilities of sharks. Visit: ilafranceporcher.wixsite.com