The plankton-feeding sharks hide out for nearly half the year, disappearing from surface waters at the end of autumn, said Gregory Skomal of the Massachusetts Marine Fisheries.
Scientists once debated whether the sharks hibernated on the ocean floor in winter, but satellite technology recently tracked tagged basking sharks to tropical waters in the western Atlantic, in the vicinity of the Caribbean and Bahamas, Skomal said.
This was a surprise because basking sharks were always believed to be cool-water sharks, restricted to temperate regions, he said.
Data from the Woods Hole Oceanographic Institution show basking sharks in winter travel at depths of 600 feet to a mile, staying at those depths for weeks and even months.
"In doing so, they have completely avoided detection by humans for millennia," Skomal said.
The sharks grow to more than 33 feet in length and weigh as much as 7 tons.