Chilean devil rays, thought to dwell mostly near the ocean's surface, actually are among the ocean's deepest divers, according to a new study by scientists at Woods Hole Oceanographic Institution (WHOI).

"So little is known about these rays," WHOI biologist and co-author Simon Thorrold said in a news release. "We thought they probably travelled long distances horizontally, but we had no idea that they were diving so deep. That was truly a surprise."

Researchers used satellite transmitting tags to track the movement patterns of 15 Chilean devil rays (Mobula tarapacana) in the central North Atlantic Ocean during 2011 and 2012. The tags, which stay attached for nine months, also measured water temperature, depth and light levels of the waters.

"Data from the tags gives us a three-dimensional view of the movements of these animals, and a window into how they're living in their ocean habitat - where they go, when, and why," Thorrold added.

Devil rays, which can grow as large as 13 feet across (four meters), are typically nomadic marine creatures, traveling vast distances across the ocean. Tag data revealed that despite previous belief that they preferred warm, shallow waters, these rays frequently took deeper dives - descending at speeds up to 13.4 mph to depths of almost 1.24 miles (2,000 meters) in water temperatures less than four degrees Celsius (39.2 degrees Fahrenheit).

When Devil rays take their trips to the ocean deep, it's usually for a total of 60 to 90 minutes - tagged rays made these descents only once every 24 hours - or for as long as 11 hours, staying around 3,280 feet (1,000 meters) below the surface.

Scientists don't actually know for sure what is attracting the rays, compelling them to go to such depths, but the WHOI team suspects large numbers of fish are the reason.

"Ultimately, answering whether these animals depend on the deep layers of the ocean for their feeding and survival could have major implications for their management and that of oceanic habitats," added co-author Pedro Afonso.

The study findings were published July 1 in the journal Nature Communications.