Hawaiian Islands Humpback Whale National Marine Sanctuary today released rare video, taken with a drone camera, showing a humpback whale with its tail out of the water, seemingly catching the wind like a sail.

In the video, a humpback mother and calf are observed swimming and diving in the sanctuary off Maui’s leeward coast last month. At one point, the mother appears to be doing a headstand and drifting with her tail out of the water.

Scientists said tail-sailing is fairly common among southern right whales but has rarely been observed or documented among humpbacks.

“We’re not entirely sure why the whales do this,” said Ed Lyman, resource protection specialist for Hawaiian Islands Humpback Whale National Marine Sanctuary in a news release. “But we think this could be another way for them to rest, nurse, or just try to stay cool. More observations will be needed to confirm this theory.”

The video was captured by a team of whale scientists and marine mammal response managers organized by the Collaborative Center for Unmanned Technologies. The scientists were trying to determine if small, remotely piloted aircraft can be used for whale research and disentanglement efforts in the sanctuary.

During the two-week study, the research team observed more than 15 humpback whales, including mothers and calves and competitive groups of males chasing after individual females.

There are strict regulations for the approach of whales and untrained public use of unmanned aerial equipment near wildlife. The drones in the Hawaii test were operated by trained professionals under approved NOAA and FAA permits, and the staging vessel was kept 100 yards away from the whales at all times.