The Bloop: Ocean's Unsolved Sound Mystery

It starts with a sound so loud, so strange, it rattled the entire scientific community. In the summer of 1997, deep under the Pacific Ocean, the U.S. National Oceanic and Atmospheric Administration—NOAA—caught something on their hydrophones that nobody could explain. It was dubbed “the Bloop.” And for years, nobody could agree on what exactly was out there in the darkness, making the ocean’s loudest unsolved noise.

Here’s the deal: the Bloop wasn’t just a weird blip on some scientist’s radar. This thing was colossal. NOAA recorded it using a network of underwater microphones positioned across the Pacific—a network originally built by the U.S. Navy during the Cold War, designed to track Soviet submarines. In 1997, those same sensors picked up a massive, ultra-low-frequency noise that rapidly rose in pitch over about a minute. The Bloop was so powerful it set off sensors over 5,000 kilometers apart—think from Boston to London, and then some.

Triangulation placed the Bloop’s source at roughly 50 degrees south, 100 degrees west—an empty patch of ocean west of the southern tip of South America. That is about as remote as it gets on this planet, a place where the ocean is colder, deeper, and more mysterious than anywhere else.

Now, here’s where things get wild. The Bloop’s frequency profile resembled that of a living creature. But it was so much louder than the loudest animal known—the blue whale—that some began to speculate about what could possibly make a noise like this. For context, the blue whale’s call can reach 188 decibels and travel for hundreds of miles. The Bloop? Scientists say it was several times louder. The Atlantic called it “house-shakingly loud.”

So, was it a massive sea monster? That became the internet’s favorite theory in the early 2000s. People wondered if this was finally proof of a gigantic, undiscovered creature—something lurking in the ocean’s trenches, making sounds louder than any living thing we’ve ever found. It was the premise of a thousand blog posts, endless Reddit threads, and even inspired stories about Cthulhu-sized cryptids.

But the Bloop isn’t the only unexplained noise NOAA has picked up. In 1991, six years before the Bloop, scientists first detected another enigma: the Upsweep. Unlike the short, sharp burst of the Bloop, Upsweep is a series of long, narrow-band sounds—almost like musical notes—lasting several seconds each. It’s been detected every year since, but only during certain seasons, peaking in the spring and autumn. NOAA traced the source to about 54 degrees south, 140 degrees west—again, deep in the middle of nowhere, between New Zealand and South America. The leading theory? Some suspect underwater volcanic activity, but even now, nobody’s pinned it down.

Then there’s Julia. Recorded on March 1, 1999, Julia lasted about 15 seconds, and it was loud enough to be picked up across the entire Equatorial Pacific hydrophone array. NOAA could only narrow the location to somewhere between Bransfield Straits and Cape Adare in Antarctica. The source? Officially, it’s still unknown, but some scientists pointed to a large iceberg running aground. What’s eerie is how quickly the sound faded after that first recording—almost as if whatever made the noise just…stopped.

Slow Down is another. On May 19, 1997, hydrophones captured a sound that slowly dropped in frequency over seven minutes. The name fits. Theories suggest a massive iceberg scraping against the seafloor, but again, there’s no direct evidence. What’s certain is it’s been picked up several times a year since 1997, always with the same bizarre acoustic signature.

And then there’s Sea Train. On March 5, 1997, hydrophones caught a sound that rises to a stable frequency, and then just hangs there—a kind of underwater drone. As of the latest official reporting, the source remains unidentified. Some believe it’s the grinding of an ice mass; others, that it’s an unknown natural phenomenon.

Each of these sounds comes with its own set of mysteries and theories. Over the years, scientists have managed to explain a few once-unidentified noises. For example, the infamous “bio-duck” sound, once a mystery, was eventually traced to the Antarctic minke whale. But the biggest mysteries—like the Bloop, the Upsweep, and Slow Down—have fended off easy answers.

Let’s go back to the Bloop for a moment. In 2012, after years of speculation, NOAA scientists released a statement saying the Bloop’s signature was “consistent with noises generated by icequakes,” that is, massive icebergs cracking and breaking away from Antarctic glaciers. When iceberg A53a disintegrated near South Georgia Island in 2008, its breakup generated sounds on hydrophones that closely matched the Bloop’s signature. But here’s where the debate lingers: some researchers and curious internet sleuths still question whether the Bloop was really just ice. The fact that it was so much louder than any known icequake has left the door open for doubt—and for imagination.

Other sounds, like Upsweep, continue to defy complete explanation. While the volume of Upsweep has been gradually declining since 1991, it still shows up seasonally, its source hidden somewhere in the abyss. And Slow Down, with its annual, unchanging signature, still crops up on hydrophones a couple times a year, like a ghostly refrain.

Why do these mysteries matter? For one, they remind us just how unexplored our own planet remains. The ocean covers over 70% of the earth’s surface, yet we’ve mapped only a tiny fraction of its depths. Each unexplained sound is a reminder that something is still out there—something bigger, weirder, and perhaps a little bit scarier than we can imagine.

And here’s the kicker: as of the most recent reporting, some of these unexplained sounds—like Upsweep and Slow Down—continue to be detected every year. Even as sensors get more sophisticated and scientists get better at identifying new natural phenomena, these old mysteries haven’t gone away. The hydrophones just keep listening, and sometimes, the abyss answers back.

So, what made the Bloop? If it really was just an iceberg, how did it get so loud? And if it wasn’t—what else is waiting for us in the deep?