Dr. Robert Dziak, NOAA’s Pacific Marine Environmental Laboratory, Ocean Acoustics Project
In 2015, NOAA and partner scientists deployed a hydrophone to a depth of 10,971 meters (6.71 miles) within the Challenger Deep trough in the Mariana Trench near Micronesia. Using the hydrophone, researchers sought to establish a baseline recording the ambient sound levels at the deepest point in the global ocean in order to:
Engineers at NOAA’s Pacific Marine Environmental Laboratory built the one-of-a-kind deep ocean hydrophone to withstand the extreme hydrostatic pressures (more than 16,000 pounds per square inch, compared to the 14.7 PSI in the average home) at the ocean depths encountered at Challenger Deep. Because of the incredible atmospheric pressure at these depths, scientists had to drop the hydrophone mooring down through the water column at no more than five meters per second to be sure the hydrophone, which is made of ceramic, did not crack.
Researchers deployed the hydrophone from the Guam-based U.S. Coast Guard Cutter Sequoia in July 2015. The device recorded deep-ocean ambient sound levels in the 10–32,000 Hz range continuously over 23 days. However, scientists had to wait until November to retrieve the hydrophone due to ship schedules and persistent typhoons. The device remained anchored to the seafloor until scientists returned.
Once they had a listen, researchers from NOAA, Oregon State University, and the U.S. Coast Guard were surprised by how much they heard.
Instead of being one of the quietest places on Earth, scientists found that at the deepest part of the ocean, there is almost constant noise. The ambient sound field is dominated by the sound of earthquakes, both near and far, as well as distinct moans of baleen whales and the clamor of a category 4 typhoon passing overhead. The hydrophone also picked up sound from ship propellers, as Challenger Deep is close to Guam, a regional hub for container shipping with China and the Philippines.
Human-created noise has increased steadily in recent decades. Establishing a baseline for ambient noise will allow scientists to determine if noise levels in the ocean are growing and how this might affect marine animals, such as whales, dolphins, and fish that use sound to communicate, navigate, and feed.