A faint pop punctuated the sound of crashing waves—the first hint something was amiss.

Sitting on board the R.V. Falkor in December 2014, David Barclay heard the sound through headphones plugged into an underwater microphone on the ship's hull. His mind flashed to the pair of scientific instruments sinking through the water beneath his feet, en route to an abyss in the Pacific Ocean known as Challenger Deep. The spot lies nearly seven miles below the waves—more than a mile deeper than Mount Everest is tall—making it the ocean's deepest point.

The two instruments were part of Barclay's work to create a compact and less expensive way to record the underwater soundscape, a project