During the so-called "Big Expedition" of 1970, Roy Mackal, a biologist who taught for 20 years at the University of Chicago, devised a system of hydrophones (underwater microphones) and deployed them at intervals throughout the loch. In early August a hydrophone assembly was lowered into Urquhart Bay and anchored in 700 feet of water. Two hydrophones were secured at depths of 300 and 600 feet. After two nights of recording, the tape (sealed inside a 55 gallon steel drum along with the system's other sensitive components) was retrieved and played before an excited LNPIB.
"Bird-like chirps" had been recorded, and the intensity of the chirps on the deep hydrophone suggested they had been produced at greater depth. In October "knocks" and "clicks" were recorded by another hydrophone in Urquhart Bay, indicative of echolocation. These sounds were followed by a "turbulent swishing" suggestive of the tail locomotion of a large aquatic animal.
The knocks, clicks and resultant swishing were believed to be the sounds of an animal echolocating prey before moving in for the kill. The noises stopped whenever craft passed along the surface of the loch near the hydrophone -- and resumed once the craft reached a safe distance. In previous experiments, it was observed that call intensities were greatest at depths less than 100 feet.
Members of the LNPIB decided to attempt communication with the animals producing the calls by playing back previously recorded calls into the water and listening via hydrophone for results, which varied greatly. At times the calling patterns or intensities changed, but sometimes there was no change at all. Mackal noted that there was no similarity between the recordings and the hundreds of known sounds produced by aquatic animals.
"More specifically," he said, "competent authorities state that none of the known forms of life in the loch has the anatomical capabilities of producing such calls."
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