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The Bloop

Donate This mysterious sound captured by NOAA hydrophones was not a sea monster.  

by Brian Dunning

Filed under Cryptozoology, General Science

Skeptoid Podcast #177
October 27, 2009
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The Bloop

Important update: Since this episode was originally published, NOAA has identified the Bloop.

It came from the depths of the South Pacific. Throughout the summer of 1997, a sound never before recorded burst from the abyss. News agencies scrambled; was this some new leviathan, an unknown monster from the deep? Nobody knew, and though this recording has taken its place among the permanent fixtures of the museums of the strange, the Bloop has never been identified.

The Bloop was on the loud side, to be sure. It was picked up on multiple sensors as far as 5,000 kilometers away. By triangulation, we know it came from somewhere right around 50°S, 100°W, which is about 1,750 kilometers west of Chile in the South Pacific. It's about as remote as you can get in any ocean. There are no islands or anything anywhere near it. The water is deep there, very deep, averaging around 4,300 meters. The Bloop was recorded several times during 1997, on the Eastern Equatorial Pacific autonomous hydrophone array, which was deployed in May, 1996 by NOAA (the National Oceanic and Atmospheric Administration) for long-term monitoring of seismic events on the East Pacific Rise.

Here's the Bloop sound, as released by NOAA and widely available on the Internet (Update: the Bloop has since been identified as an icequake):

But the version released by NOAA has been sped up 16 times. I tried and tried to find an original realtime version, but could not. A few people have taken the sped-up version and slowed it down to 1/16th speed, but the results are usually pretty bad. 93% of the data is gone, so expectations have to be pretty low. Here is the best realtime version I could find, but be aware it's low frequency and you might not be able to hear it without a good subwoofer:

Cryptozoologists love the Bloop, because to them it is evidence pointing to the existence of a gigantic unknown creature. Virtually every web page about the Bloop (and there are a lot of them) repeats this same quote, word for word:

Though it matches the audio profile of a living creature, there is no known animal that could have produced the sound. If it is an animal it would have to be huge — much larger than even a Blue whale, according to scientists who have studied the phenomenon.

I've been unable to find the original source of this quote. Only one website on which I found it gave an attribution, saying it was a quote from NOAA. However this text does not appear on any NOAA websites, and its wording is inconsistent with NOAA's typical descriptions of unknown sounds. When a sound is unknown, NOAA says it's unknown and leaves it at that; it is not their habit to editorialize or hypothesize about giant cryptids.

So, for now, the identity of these "scientists who have studied the phenomenon" remains a mystery. Many cryptozoologists have written about the Bloop, as is easily shown by a simple Google search; but from what I could tell, few legitimate zoologists have, and none who have concluded the Bloop has a biological origin.

The Bloop
"The Bloop" (0-50 Hz, 16×)
Image credit: NOAA
Alaska Humpback
Humpback Whale (0-500 Hz, 10×)
Image credit: NOAA
North Pacific Blue Whale
N Pacific Blue Whale (0-50 Hz, 10×)
Image credit: NOAA
Atlantic Finback Whale
Atlantic Finback Whale (0-50 Hz, 10×)
Image credit: NOAA
Atlantic Minke Whale
Atlantic Minke Whale (0-50 Hz, 10×)
Image credit: NOAA
South Pacific Blue Whale
S Pacific Blue Whale (0-40 Hz, 10×)
Image credit: NOAA
Volcano (0-40 Hz, 16×)
Image credit: NOAA

Fortunately, we are not entirely without our own resources to test this suggestion that the Bloop "matches the audio profile of a living creature". There are three basic types of sounds in the oceans: Natural sounds like volcanoes and earthquakes, biological sounds from sea creatures, and man-made sounds from boats or other machinery. Usually you can take your unknown sound and compare it to a selection of known sounds, and get a pretty good idea what it is. Sounds can be represented on colorful graphs called spectrograms. Time is one dimension, and frequency is the other. The amplitude is represented by the color. An ongoing sound with a certain frequency range, like a boat engine, creates a solid band across the spectrogram. A chirp from a dolphin would make a little streak. This gives us a two-pronged approach to trying to match the Bloop to a known source: We can listen to it to get a subjective feel for it, and we can also compare its spectrogram to known spectrograms to get a firmer, more quantifiable comparison.

Let's take a look at the Bloop's spectrogram. The first thing you notice about the Bloop is that it's a pretty broad spectrum sound. For its duration, it pretty much covers the frequencies from about 10 to 45 Hz. It's a well-defined vertical smudge. If we take a look at the spectrograms for some other known sources, we should be able to pick out which sounds have this same general profile. The Atlantic Finback whale does. It makes a series of chirps, and each chirp looks visually quite similar to the Bloop. Here's what it sounds like:

That sound is sped up also, 10×. All of the sounds we're going to listen to have been sped up by NOAA, since they're mostly subsonic in real time. Whales are no exception. These Finback chirps are between 20 and 30 Hz, at the extreme lowest end of what humans can hear. Note that the Finback's 10 Hz spread is much narrower than the Bloop's 35 Hz spread. We might expect this if the Bloop is indeed some unknown gigantic sea creature. But when we listen to the sounds subjectively, it's easy to hear that the Finback has a clear voice-like tone, while the Bloop simply sounds like bubbles. Quite different.

Another possible candidate is the South Pacific Blue whale. Its spectrogram also makes a vertical smudge for each syllable (if we can use the word syllable). But the Blue's smudges are less well defined, and have clear bands of dominant frequencies. Have a listen:

Here's another Blue, this one from the North Pacific:

This one's spectrogram is not a single vertical stroke, instead it's a series of short horizontal strokes stacked in a vertical column. It vocalizes on several frequencies at once. But interestingly, both Blue whales cover the same frequency range as the Bloop, about 10 to 45 Hz. While that's interesting, it only tells us so much. First, the sounds are clearly different: the Bloop is noisy, the whale is a clear tone. Second, a lot of things in the sea are in that range. Here is the sound of a small ship (again, sped up 10×), and it's also about 10 to 45 Hz:

But there are major differences between the ship and the Bloop. The ship is a constant tone, while the Bloop is a single, well, bloop.

Some large creatures are well outside of 10 to 45 Hz. The Humpback whale is just one example. Its range goes from the subsonic all the way up to about 450 Hz:

And like the other whales, the Humpback has a clear tone in its voice. Even the whales who have weird patterns have a distinct voicelike tone. Listen to this Atlantic Minke whale:

Another thing that I notice is that whales don't just make a single chirp and are then silent for weeks or months. All the whales have songs, or sentences, whatever you want to call them. They vocalize with a series of sounds, like every animal I can think of; they do it frequently, and they do it every day. This is, to my mind, the most significant difference between known vocalizations and the Bloop. If the Bloop is made by an animal, it's an animal that only had a few words to say, ever, in its whole life, and it said a single word once a month or so for part of 1997, and then never said anything else in more than a decade. This is just not the way animals vocalize. This doesn't prove anything, but it's meaningful enough that, based on this alone, I would probably put my money on the Bloop being anything other than a living creature. Not a conclusion; just a hunch.

Earthquake (0-60 Hz, 16×)
Image credit: NOAA
Ship (0-50 Hz, 10×)
Image credit: NOAA
"Julia" (0-50 Hz, 16×)
Image credit: NOAA
"Train" (0-50 Hz, 16×)
Image credit: NOAA
"Upsweep" (0-40 Hz, 20×)
Image credit: NOAA
"Slowdown" (0-40 Hz, 16×)
Image credit: NOAA

Not that that helps narrow it down much. The Bloop is a pretty poor match for the natural sounds we have recordings of. Listen to this undersea earthquake:

That's a horrible match both audibly and visually. Clearly it doesn't sound anything like the Bloop, and the earthquake's spectrogram is a hazy, blurry wedge, starting out with full-spectrum noise from 0 to 60 Hz, and fading out down to just 0.

Volcanic activity is also a bad match. Listen to this undersea volcanic eruption, and remember, like the Bloop, this is also sped up 16×:

The frequency range is pretty close to the Bloop, say 10 to 40 Hz, but it has no clear distinct pattern, it's just a lot of vague noise.

So, while there are substantial differences between the Bloop and anything else we can think of, the Bloop is not really all that unique. Sources that promote it as some kind of rare mystery often fail to point out that NOAA libraries contain lots of sounds that are unidentified, and the Bloop is just one. Many of these have spectrograms that also consist of clean, well-defined shapes. Listen to this one, nicknamed the Train, with a whistle at about 35 Hz (Update: the Train has since been identified as an iceberg dragging along the sea floor):

Or this one, nicknamed Julia. Tell me this doesn't sound like a weird vocalization (Update: Julia has since been identified as an iceberg grounding on the sea floor):

If oceanographers classify Julia as unknown, with its relatively obvious similarity to a whale, it's less surprising that the Bloop is so classified. Here's another one whose spectrogram looks quite a lot like the Finback whale, and it seems to be chattering along quite happily. NOAA calls this one the Upsweep (Update: Additional information has been posted about the Upsweep, and it may be related to volcanic activity):

Here's another called the Slowdown. Slowdown has a clear voice and a clear spectrogram, much more like a whale than the Bloop (Update: Slowdown has since been identified as an iceberg grounding on the sea floor):

So, be skeptical of news reports or websites that parrot the unsourced claim that the Bloop "matches the audio profile of a living creature." It doesn't really; and there are other unknown sounds that sound a lot more like an animal. And I can think of half a dozen phenomena off the top of my head for which I couldn't find recordings or spectrograms, that might be good candidates to explain the Bloop. What about undersea releases of natural gas bubbles? What about deep sea vents? What about some kind of manmade activity: A submarine blowing a ballast tank or releasing some undersea weapon or machine?

I make no pretense at being able to explain the Bloop. NOAA experts far more knowledgeable than I have been unable to classify it. What I do know, and what I encourage you to consider, is that no good explanation is very well supported; certainly not the pop-culture version that CNN would have us jump to, that it's a giant sea monster. Sometimes the only way to be right is to simply say "I don't know...yet."

Important update: Since this episode was published, NOAA has identified the Bloop.

By Brian Dunning

Please contact us with any corrections or feedback.


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Cite this article:
Dunning, B. "The Bloop." Skeptoid Podcast. Skeptoid Media, 27 Oct 2009. Web. 19 Apr 2024. <>


References & Further Reading

National Research Council. Ocean Noise and Marine Mammals. Washington, DC: National Academy of Sciences, 2003. 27-82.

Nemiroff, R., Bonnell, J. "Astronomy Picture of the Day." APOD. NASA, 27 Apr. 2010. Web. 9 Sep. 2010. <>

NOAA. "Sounds in the Sea 2001." NOAA Ocean Explorer. National Oceanic and Atmospheric Administration, 12 Jul. 2005. Web. 15 Oct. 2009. <>

Richardson W., Greene Jr., C., Malme C., Thomson D. Marine Mammals and Noise. San Diego: Academic Press, Inc., 1995.

Wille, Peter C. Sound images of the ocean in research and monitoring. New York: Springer, 2005.

Wolman, D. "Calls From the Deep." New Scientist. 15 Jun. 2002, Issue 2347: 35.


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