A sound recorded by NOAA in 1999, lasting about fifteen seconds, loud enough to appear on every hydrophone in the Equatorial Pacific array.
It is a large iceberg running aground on the seafloor off Antarctica.
FigureDrawn by Anomaly Registry. NOAA attributes the sound to a large iceberg grounding on the seafloor between the Bransfield Strait and Cape Adare. The array detects tens of thousands of ice-generated sounds in the Southern Ocean every year.
What is documented
The recording. On 1 March 1999, the NOAA Equatorial Pacific autonomous hydrophone array recorded a sound of roughly fifteen seconds' duration. It was picked up across the whole array, which means it carried a very long way.
The source. It was triangulated to the Southern Ocean, in the region between the Bransfield Strait and Cape Adare: Antarctic waters, and a known source of cryogenic signals.
The attribution. NOAA attributes it to a large iceberg grounding: an immense mass of ice making contact with the seabed and dragging across it.
Leading explanations
Ice, running aground. Established, and unremarkable.
Icebergs of the size involved are enormous. When one touches the seafloor and begins to scrape, it generates a great deal of acoustic energy at low frequency, and seawater carries low frequency extremely efficiently.
NOAA's hydrophones detect tens of thousands of ice-generated sounds in the Southern Ocean every year. Julia is one of the loud ones.
What the popular version gets wrong
"Julia is an unexplained ocean sound." It is not, and it never really was. It was attributed by the agency that recorded it, using the same reasoning and the same evidence that resolved the Bloop.
"It is one of the great mysteries of the deep." It is a piece of ice, of enormous size, touching the bottom. That is a genuinely extraordinary thing to be able to hear from the other side of an ocean, and it is not a mystery.
Current status
Later explained. The registry files this record briefly, because there is very little to say, and the standard requires that a thin case receive a thin entry. Padding it would mean speculating, and there is nothing here to speculate about.
It is ice.
Sources
NOAA Pacific Marine Environmental Laboratory, Acoustics Program: recording of 1 March 1999 and subsequent attribution.
Antarctic hydrophone data on cryogenic signals and iceberg grounding.
Last reviewed: July 2026.
Records are provisional. Where the evidence changes, the entry changes.
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