A reconstruction is not a direct recording. This composite was assembled from the Keldysh's expedition acoustic logs — partial, and recorded under field conditions — combined with archival material from Soviet oceanographic programmes that became partially accessible in the post-Soviet period. The result represents a working model of the source signal as it was detected at depth, prior to the object's surface emergence. It will be revised if primary source data becomes more complete.
What Was Detected
In August 1976, the research vessel Akademik Mstislav Keldysh detected an anomalous acoustic signal from depth during a scheduled oceanographic expedition in the Barents Sea. The signal was recorded by the ship's own passive hydrophone array and logged by expedition personnel before any visual contact with the source.
The signal fell in the 8–34 Hz range — below typical biological frequencies for Arctic species, and outside the acoustic profile of the seabed geology in the surveyed zone. Its amplitude profile was consistent with a source at significant depth, and its temporal structure — described in expedition notes as "rhythmic without being mechanical" — did not match ambient noise catalogues from the region. It was this signal that prompted the deployment of the ship's submersible.
The submersible did not return under its own power. When it was recovered, the object accompanied it to the surface. The acoustic signal ceased at the moment the object cleared the waterline. This cessation — abrupt, complete, simultaneous with emergence — is the detail that expedition leader Professor Viktor Tsarev marked as the primary research question in his field notation.
The Cessation
The most significant documented feature of the 1976 Barents Sea signal is not its frequency profile or its amplitude — it is the timing of its end. Expedition acoustic logs record the signal terminating at the moment the recovered object broke the surface. Not before. Not after. The cessation is described in Professor Tsarev's notation as "instantaneous — within the resolution of our instruments."
This is not a characteristic of geological acoustic sources, which do not respond to surface events. It is not a characteristic of biological sources. It does not correspond to any signal class in the expedition's reference documentation. The cessation is the element of the acoustic record that resists every available explanatory framework — and that, in Tsarev's own words, "demands a hypothesis I am not yet prepared to write."