In an hours-long struggle, as many as 70 killer whales hunted down and killed a blue whale off the southwestern coast of Australia, according to a marine biologist who saw the “astonishing, a little bit disturbing and truly mind blowing” event take place.

At first, it seemed like a normal day of whale watching, said Kristy Brown, a marine biologist with Naturaliste Charters, a company that runs whale-watching tours in Western Australia. The vessel happened upon two pods of orcas in Bremer Bay Canyon, about 28 miles (45 kilometers) off the coast, that were “playing and surfing the waves,” Brown wrote in a March 16 blog post.