Spotted off the coast of the Azores in Portugal, this incredible image shows a pod of sleeping sperm whales. Back in 2008, Patrick Miller, a senior researcher fellow at the University of St. Andrews Gatty Marine Research Institute, and his colleagues affixed suction cups with data-logging tags onto 59 sperm whales at various open-water locations worldwide. The tags allowed the scientists to monitor the whales’ movements 24/7.

The researchers, whose study was published in Current Biology, noticed the whales performed the mesmerizing drift dives 7.1% of the time, usually between 6 p.m. and midnight. The scientists observed two types of drift dives. The first, head-up drift dives, happen when a whale’s rear end slowly sinks into the water from a horizontal posture. During the second type, head-down, the whale descends slowly with its head directed toward the ocean floor. It travels downward about one or two body lengths in depth before flipping back upward toward the water’s surface. The researchers think the whale’s internal buoyancy causes this natural upward motion, similar to how a sinking apple eventually bobs back to the surface.

“Because the drift dives are quite short (averaging around 12.7 minutes in duration) and are broken by the need for the whale to move to the surface to breathe, it seems that they sleep over short interrupted periods,” said Miller. [Source: Discovery News]