The original signal propagated at frequencies below the threshold of human hearing. Any public version of this recording has been speed-shifted by a factor of approximately 16 to bring the frequency content into audible range. This is a standard presentation technique for infrasonic events — the resulting sound is a representation of the waveform structure, not the signal as it was detected.
Recording Conditions
The recording was made by NOAA's Pacific Marine Environmental Laboratory hydrophone array — Cold War submarine-detection infrastructure partially declassified in the early 1990s and repurposed for oceanographic science. The array positions hydrophones in the SOFAR channel: a deep-water acoustic waveguide where sound travels with minimal attenuation over intercontinental distances.
The 1997 event triggered multiple geographically dispersed stations simultaneously. This multi-station detection is what allows source triangulation and rules out local artefact. The detection range exceeded 5,000 kilometres — consistent with an extremely energetic source or unusually efficient SOFAR coupling. Peak amplitude exceeded any other biological or geological event in NOAA's acoustic record.
On the Icequake Classification
NOAA classified the Bloop as a probable icequake event in 2012 — the acoustic signature of a large-scale ice fracture in the Antarctic convergence zone. The classification is plausible and has been widely accepted. It is not, to date, verified against a confirmed icequake event with a matching frequency-rise profile and equivalent amplitude. The related archive file CG-058 (Icequakes) contains the classification history and the open questions it leaves unaddressed. The audio record AUDIO-001 is maintained separately from the main archive grid.