You might think it would be hard for one of the world’s largest fish to hide for months at a time. But the whereabouts of the 32-foot (10-meter) long basking shark for half the year have long been a mystery to humans.

The massive sharks, aka Cetorhinus maximus, are a fairly common sight along the East Coast of the U.S. in summer and fall. But then they practically vanish.

Researchers assumed the sharks, which peacefully feed on plankton near the sea surface much of the year, were simply hibernating offshore. Not so, it turns out.

Sharks tagged off the coast of Massachusetts headed south—way south—for the winter. Most cruised down to the Caribbean, and some ventured as far as Venezuela and the mouth of the Amazon River, says a study published earlier this month in Current Biology.

Researchers used satellites to track tagged fish into the surprising waters—and depths. Some of the tagged sharks spent as many as five months more than 650 feet (198 meters) below the surface.

The findings "absolutely surprised" the team, Gregory Skomal, an aquatic biologist at Massachusetts Marine Fisheries and lead study author, said in a statement.

The results mean that what were previously believed to be distinct basking shark populations, which are already marked as "threatened" by the International Union for Conservation of Nature, might be a single, even smaller population.

Image of the basking shark, which can weigh more than 15,432 pounds (7 metric tons), and divers (in the background) courtesy of Chris Gotschalk via Wikimedia Commons