"Basically, the sperm whales had huddled together like logs, creating a protective wall against the orcas." -- Shawn Heinrichs "Oh my God, it's orcas attacking sperm whales." -- Shawn Heinrichs A pod of five orcas had surrounded a half-dozen sperm whales. "We saw this huge dorsal fin cut through the water." -- Shawn Heinrichs "Aren't we going to get in the water?" -- Shawn Heinrichs "There was a frothing, dark pile of shapes ahead of me." -- Shawn Heinrichs "The orcas would be slamming into their sides, taking nips of these poor sperm whales." -- Shawn Heinrichs "The sperm whales would roll over and stick their heads out of the water, gasping." -- Shawn Heinrichs "They were hunting." -- Shawn Heinrichs Video shot from the surface. Brett Heinrichs/YouTube

If killer whales lived on land, we'd be in trouble. Highly intelligent and social, the black-and-white marine mammals hunt in packs, launching coordinated attacks on other whales and sharks, and even wave-wash seals off Antarctic ice floats.On April 18, a half-dozen orcas battled a pod of sperm whales off the southern coast of Sri Lanka. The unusual encounter is one of fewer than a dozen such recorded conflicts – and the first observed in the Indian Ocean.

It's also the first to be captured in underwater photos and video (underwater video not yet available, above-water video in last slide).

"Killer whale attacks on sperm whales have rarely been observed or reported," said Robert Pitman, a marine ecologist with NOAA's Southwest Fisheries Science Center. "My guess is that hunting sperm whales is not something to be taken lightly by killer whales, and that worldwide, there may only be a few groups that have mastered the ability to prey on sperm whales and their young."

Adult sperm whales are larger and can dive much deeper – and for longer – than killer whales. And their strong tails are a hazard to predators.

"A sperm whale tail crashing down on a killer whale would be a potentially life-threatening experience," Pitman said. "On the other hand, despite their giant size and impressive teeth, sperm whales are actually quite timid and terrified in the presence of killer whales, and, amazingly, they don't seem to directly target killer whales with their tail slapping."

Instead, it appears that when attacked, sperm whales cluster together. That's what Shawn Heinrichs observed off the Sri Lankan coast last week.

"We saw the water churning on the horizon," said Heinrichs, a photographer and filmmaker who was in the area looking for blue whales. He and his colleagues steered their boat toward the patch of white water. As they got closer, they saw an enormous dorsal fin slicing through the water – a killer whale trademark – and then noticed the group of sperm whales, clustered together in a defensive stance.

At that point, Heinrichs did what many of us would not do: He jumped in.

"I grabbed my camera and slid off the side of the boat," he said. "There was a frothing, dark pile of shapes ahead of me. When I drifted away from the boat, the largest orca in the pod made a beeline for me but veered off at the last moment and dove deep."

For about 30 minutes, the five orcas repeatedly slammed into the half-dozen sperm whales, bashing and nipping at them, leaving the waters murky and bloody. "It was this pile of chaos," Heinrichs said.

When the encounter ended, it appeared as though the orcas may have separated a juvenile sperm whale from the pod, and then had either drowned it or eaten it, Heinrichs said. "Whether they got it or not, I couldn't tell."

All photos: Shawn Heinrichs, Blue Sphere Media

Editor's Note: This story was updated at 8 p.m. with comments from Robert Pitman.