Decoding the mysterious sounds from the world's deepest ocean trench

New research reveals the exact origin of a mysterious biotwang in the Pacific Ocean, first detected near the Mariana Trench.

Researchers have finally determined the source of the mysterious noise coming from the world's deepest ocean trench. The strange sound, called a "biotwang," is actually the call of a Bryde's whale ( Balaenoptera edeni ) . This whale species can use the call to locate other whales. Scientists first discovered the unusual noise in 2014 while using an underwater glider to conduct an acoustic survey of the Mariana Trench. This is the deepest ocean trench on Earth, stretching more than 2,400 kilometers (1,500 miles) south of Japan and has a maximum depth of 10,935 meters (35,000 feet), according to Live Science .

Bryde's whale hunts for sardines underwater. (Photo: Scuba Magazine).

The biotwang can be divided into two distinct parts, the first a low, mumble-like sound that resonates in deep water, and the second a high-pitched metallic sound that researchers have likened to the sounds of spaceships in Star Trek and Star Wars. These sounds initially puzzled scientists. But in 2016, a team of researchers speculated that biotwangs were more likely calls from large baleen whales such as blue whales ( Balaenoptera musculus ) or humpback whales ( Megaptera novaeangliae ). However, the sound did not match the calls of any known whale species.

In a new study published on September 18 in the journal Frontiers in Marine Science, researchers were finally able to prove that Bryde's whales were making the noise, thanks in part to a new artificial intelligence (AI) tool that sifted through more than 200,000 hours of recordings, including a variety of ocean sounds. The team suspected that Bryde's whales were behind the biotwang when they saw 10 whales swimming near the Mariana Islands and filmed nine of them making the distinctive noise.

"Once or twice it's a coincidence, but nine times it's a Bryde's whale ," said study leader Ann Allen, an oceanographer at the National Oceanic and Atmospheric Administration's (NOAA) Pacific Islands Fisheries Science Center.

But to prove that Bryde's whales were vocalizing, the team matched the noise to the animals' migration patterns, cataloguing years of recordings collected by monitoring stations across the Mariana Islands and surrounding areas. They sped up the process by using AI to turn biotwangs into images, called spectrograms, that machine learning algorithms could easily distinguish from other noises.

The study also found that biotwangs were only audible in the northwest Pacific Ocean, even though Bryde's whales live over a wider area, suggesting that there is only one specialized population of whales making the sounds. The data also revealed a spike in biotwangs in 2016, when rising ocean temperatures due to an El Niño event increased the number of Bryde's visiting the area. The researchers are still unclear as to why the calls sound so strange. They will need more data to be sure.