Anomaly•Registry
AR-0041

Slow Down

Later explained
Date 19 May 1997
Location Equatorial Pacific
Coordinates -15.0000, -115.0000
Category Sounds and Signals

Summary

A sound recorded by NOAA in 1997 that descends slowly in frequency over about seven minutes. It was heard on hydrophones two thousand kilometres apart.

The name describes what it does. It is ice.

19 May 1997. the frequency falls for seven minutes. high low 0 min 7 min a gradual descent over seven minutes is what a mass of ice does as it comes to rest. see AR-0004 and AR-0040.
Figure Drawn by Anomaly Registry. The signal fell steadily in frequency across roughly seven minutes and was detected on hydrophones some 2,000 km apart. NOAA attributes it to ice: most probably a large iceberg coming to rest against the seabed.

What is documented

The recording. On 19 May 1997, the NOAA Equatorial Pacific autonomous hydrophone array recorded a signal that fell steadily in frequency over a period of roughly seven minutes.

The range. It was detected on sensors separated by around two thousand kilometres.

The attribution. NOAA attributes it to ice: most probably a large iceberg running aground and then losing contact with the seabed, or the friction of a great mass of ice moving against rock.

Leading explanations

Ice. Established, and it is the same family as AR-0004, AR-0040 and the sound NOAA calls the Train.

A very large body of ice grinding against the seabed produces a sustained, low-frequency signal. As the contact changes, as the berg slows, shifts, or breaks free, the frequency changes with it.

A gradual descent over seven minutes is what a mass of ice does as it comes to rest.

What the popular version gets wrong

"Slow Down has never been explained." NOAA attributed it, along with Julia and the Train, to the movement of ice in the southern seas, using the same evidence that resolved the Bloop.

"It sounds biological." So did the Bloop, and for the same reason: a slowly varying frequency resembles the frequency sweep of an animal call, and ice, moving against rock under thousands of tonnes of pressure, varies slowly.

Current status

Later explained. Attributed by NOAA to ice.

The registry files this record briefly, and states plainly why. There is not much in it. A longer entry would not contain more evidence; it would contain more of the registry's own words, and that is not what an archive is for.

Sources

  • NOAA Pacific Marine Environmental Laboratory, Acoustics Program: recording of 19 May 1997 and subsequent attribution.
  • Dziak, R. (NOAA and Oregon State University), on the attribution of Julia, Slow Down and the Train to ice movement.

Last reviewed: July 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.

Related records
AR-0002 The Wow! Signal A strong narrowband radio signal from Sagittarius, detected once, never repeated. 1977 Unsolved
AR-0004 The Bloop An ultra-loud ocean sound, later traced to icequakes from calving Antarctic ice. 1997 Later explained
AR-0005 The 52-Hertz Whale A real whale calling at an unusually high frequency. The species is still debated. 1989 to present Partially explained
AR-0006 Max Headroom Broadcast Intrusion Two Chicago television signals hijacked on a single night. The perpetrator was never identified. 1987 Unsolved

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