The Bloop and the Sounds of the Deep

The ocean is not silent. It is one of the noisiest environments on Earth, filled with the creaks of shifting tectonic plates, the songs of whales, the crackle of shrimp colonies, and the distant rumble of submarine volcanoes. Most of this sound is inaudible to human ears. But NOAA — the National Oceanic and Atmospheric Administration — built a network to hear it.

The Sound Surveillance System, or SOSUS, was originally a Cold War installation designed to detect Soviet submarines. After the Cold War ended, NOAA repurposed many of its hydrophones for scientific research. By the late 1990s, researchers were using the network to monitor whale populations, track undersea earthquakes, and listen for anything unusual in the deep sound channel — the layer of water at around 1,000 metres depth where acoustic energy can travel for thousands of kilometres without dissipating.

In the summer of 1997, something unusual arrived.

A Signal Without a Name

NOAA’s Pacific hydrophone network picked up an ultra-low-frequency sound — well below 20 Hz, which is the lower limit of human hearing — that was detectable across the entire Pacific. The signal lasted roughly one minute. Its amplitude was so high that it could only have been produced by something extraordinarily loud. The sound rose in frequency as it progressed, which is why researchers gave it the name they did: the Bloop.

The Bloop had characteristics that were, in the words of the original analysts, consistent with a large biological source. Whales produce low-frequency calls. Blue whales, the largest animals alive, produce calls that can be heard hundreds of kilometres away. But the Bloop was orders of magnitude louder than any known biological call. If it was an animal, it would have to be vastly larger than any creature currently known to science.

This was, inevitably, the detail that escaped into popular culture. The Bloop’s source coordinates placed it near the location Lovecraft had assigned to the sunken city of R’lyeh in his 1928 story “The Call of Cthulhu.” The internet did what the internet does. For a decade, the Bloop was one of the most reliably circulated pieces of cryptozoological evidence — proof, to some, that something ancient and enormous still moved through the deep ocean.

What Geology Revealed

In 2005, NOAA researchers published a more thorough analysis of the Bloop and several similar sounds detected during the same period. The conclusion was not a creature. It was ice.

Antarctic ice quakes — cryoseisms — occur when large masses of ice fracture under stress. When enough ice shifts at once, particularly when glaciers calve into the ocean, the resulting acoustic event can propagate enormous distances through the deep sound channel. The frequency signature of the Bloop, when compared against known cryoseism recordings, proved to be a match. The source coordinates pointed to a region of the southern Pacific near Antarctica where glacial calving events were known to occur.

The mystery was solved. But the episode left behind something more interesting than its resolution: a window into the acoustic ecology of an ocean we barely understand.

The Ocean’s Other Voices

The Bloop was not the only named anomaly in NOAA’s records. The same archive contains the Train, a sound that rises steadily in frequency over a period of several seconds and was detected multiple times through the late 1990s — also now attributed to ice. The Julia was a long, moaning sound recorded in 1999, also cryoseismic in origin. The Upsweep, first detected in 1991 and still occasionally heard, is a different case: it originates from a specific point on the Pacific-Antarctic Ridge and is thought to be connected to undersea volcanic activity, though its precise mechanism is still debated.

What these sounds collectively reveal is that the ocean floor is far more acoustically active than scientists appreciated before hydrophone networks gave them ears to hear it. Tectonic plates move, glaciers shift, and volcanoes rumble on a continuous basis — and all of that movement produces sound that propagates through the world’s oceans as a kind of geological music.

The deep sound channel itself is a remarkable phenomenon. Because sound travels at different speeds depending on water temperature and pressure, there is a layer — the SOFAR channel — where sound is refracted back and forth without losing energy to the surface or the seafloor. A large enough sound event in this channel can travel from one side of an ocean to the other. Blue whales are thought to use this channel for long-distance communication, which is why their calls evolved to be so low in frequency.

What We Still Cannot Hear

The resolution of the Bloop mystery does not resolve the broader question of what lives in the deep ocean. The hadal zone — depths below 6,000 metres — remains one of the least explored environments on Earth. We have better maps of Mars than of the ocean floor. The creatures that live in extreme deep water are adapted to conditions — pressure, darkness, cold — that make recovery and study profoundly difficult.

Giant squid were only first photographed alive in 2004. The colossal squid, the largest invertebrate on Earth, was not confirmed as a species until 1925, and has been filmed in its natural habitat only once.

The ocean’s sound archive is a record of geological events. But it is also, in some sense, a reminder of how much remains hidden. The Bloop turned out to be ice. The next anomaly may have a similarly mundane explanation. Or it may not.

Key Takeaways

• The Bloop was detected by NOAA’s Pacific hydrophone network in 1997 — an ultra-low-frequency sound loud enough to be heard across the entire Pacific Ocean.
• Initial analysis noted characteristics “consistent with a large biological source,” sparking speculation about unknown deep-sea creatures.
• In 2005, NOAA researchers concluded the Bloop was an Antarctic ice quake (cryoseism), matching known signatures of glacial fracture events.
• The SOFAR deep sound channel allows acoustic events to propagate thousands of kilometres without significant energy loss — making the ocean an enormous long-range communication system.
• Other NOAA anomalies (the Train, Julia, Upsweep) are largely explained by ice and volcanic activity, but the Upsweep’s precise mechanism remains debated.