A sound recorded by NOAA in 1999, loud enough to be heard across its whole hydrophone array in the equatorial Pacific, thousands of kilometres from where it began.
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 large numbers 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 loud, low sound. It was picked up across the whole array, which means it carried a very long way. NOAA's published recording is sped up sixteen times so that it can be heard.
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.
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 large numbers 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. 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: September 2026.
Records are provisional. Where the evidence changes, the entry changes.
Found an error? Tell us.
Related records
AR-0004
The BloopAn ultra-loud ocean sound, later traced to icequakes from calving Antarctic ice.
1997
Later explained
AR-0041
Slow DownA seven-minute descending tone, attributed to a drifting iceberg running aground.
19 May 1997
Later explained
AR-0002
The Wow! SignalA strong narrowband radio signal from Sagittarius, detected once, never repeated.
1977
Unsolved
AR-0005
The 52-Hertz WhaleA real whale calling at an unusually high frequency. The species is still debated.