When scientists collected acoustic data from an area around the Mariana Trench in the Pacific Ocean in the fall of 2014 and spring of 2015, they couldn’t identify a sound they heard with any known animal sounds. After analyzing the recording for over a year, they think the sound represents a previously unknown type of call made by baleen whales.

Between 2 and 4 seconds long, the complex sound is a five-part call that “includes deep moans at frequencies as low as 38 hertz and a metallic finale that pushes as high as 8,000 hertz. Researchers from Oregon State University, who recorded and analyzed it, named it the “Western Pacific Biotwang.”

Sharon Nieukirk, senior faculty research assistant in marine bioacoustics at the university and lead author of a recently published research paper on the subject, said in a statement: “It’s very distinct, with all these crazy parts. The low-frequency moaning part is typical of baleen whales, and it’s that kind of twangy sound that makes it really unique. We don’t find many new baleen whale calls.”

Baleen whales are a large family of whales and include many subgroups, all of which feed by using baleen plates in their mouths to filter krill and small fish from the water, instead of teeth. The calls from Mariana Trench are thought to belong to minke whales, which are a kind of baleen whales. Nieukirk and her colleagues think so because the calls of minke whales have been studied widely and the new recording is most similar to “Star Wars,” the name given to a minke whale call off the northeast coast of Australia.

But scientists are not certain at all, since they are entirely unsure about the purpose of the call. Baleen whale calls are mostly related to mating, which is why they are mainly heard in the winter, but this call was recorded round the year.

“If it’s a mating call, why are we getting it year round? That’s a mystery. We need to determine how often the call occurs in summer versus winter, and how widely this call is really distributed,” Nieukirk said in the statement.

The sounds were recorded using passive acoustic ocean gliders, which are largely autonomous instruments that can travel for months at a time and dive up to almost 3,300 feet (1 kilometer).

Mariana Trench is the deepest known place in the world’s oceans. At its lowest point of over 36,000 feet, it is more than 7,000 feet deeper than Mt. Everest is tall. It lies south of Japan and north of Australia in the western Pacific.