It's not often that people get a glimpse of the largest animal that has ever lived. But rare drone footage of a blue whale mother and her calf gliding through the Southern Ocean off Antarctica has gone viral this week.

The footage was shot in late January by members of the activist group Sea Shepherd Society, who are patrolling the area in the ship Steve Irwin to deter poachers.

Watch: Rare video of blue whales.



"This is nice footage, and what makes it interesting is that Antarctic blue whales have a low density and are hard to see and study," says John Calambokidis, a National Geographic explorer and research biologist who studies the animals with Cascadia Research in Washington state.

Highly endangered, blue whales can reach a length of up to 98 feet (30 meters) and can weigh up to 200 tons (180 tonnes). The video shows a mother and a calf that Calambokidis says is about six months old. Calves usually stay with their mothers for six to eight months, nursing. After that, they disperse and live primarily alone, unlike the more social whale species (like orcas and sperm whales).

Blue whales were nearly hunted to extinction in the first half of the 20th century and were prized for their large size. Their pre-hunting population is estimated to have been around 300,000. The majority lived around the Antarctic, but their numbers collapsed to 0.15 percent of their historic strength, or about 450 individuals.

Legal hunting of blue whales ended by international treaty in the 1960s, but illegal hunting continued until the late 1970s, largely by the Soviets, says Calambokidis. Today, some controversial whaling occurs of some other species of whales, something Sea Shepherd opposes in its campaigns (learn more about this).

"In the 1970s many scientists thought blue whales were doomed to extinction, that their population had been knocked so low that they would not be able to recover," Calambokidis notes. But since then, there have been encouraging signs of a slow recovery. (Learn about whaling in Iceland.)

Today, scientists aren't sure how many blue whales remain, but the numbers are thought to be between 10,000 and 20,000, and slowly rising. The biggest threats are accidental strikes by ships, particularly around heavy shipping lanes. In response, some shipping lanes have been changed and captains have been educated to keep a lookout. Results have been mixed. Blue whales have trouble avoiding oncoming ships due to their hugeness and their tendency to not pick up on the danger, Calambokidis says. Another threat to the animals is disruption to their feeding behavior from use of sonar in the water.

This video, however, offers some evidence that the slow-breeding animals are gradually recovering from the horrors of industrial whaling.