To reveal a gruesome dolphin slaughter to the world, the makers of the documentary The Cove used spy drones, cameras disguised as rocks and a lot of daring.

The movie depicts a hunt in the waters off Taiji, Japan, where at least 2,000 dolphins are killed every year, with a few caught and sold to aquariums. The meat, containing toxic levels of mercury is sold to people, often passed off as whale meat. Dolphin killing receives less attention than whale hunts, but many scientists say their death is every bit as tragic.

Like other cetaceans, dolphins appear capable of such high-level cognition that in some ways they might be considered people. Their neurological systems of emotion and social communication are highly developed. Some researchers think their high-pitched vocalizations may contain aspects of language. They may even have names for each other.

Among the most vocal critics of dolphin hunting and capture is Ric O'Barry, who trained the animal stars of the TV show Flipper in the 1970s. It was O'Barry who convinced National Geographic photographer Louie Psihoyos to make a film about the killing in Taiji, from which only fragmented accounts of the hunt had previously emerged.

Though the town itself is adorned with statues of dolphins, and sidewalks are embedded with tiles containing their likenesses, the killing takes place in a carefully guarded cove. Technically it's a national park, but even Japanese citizens can't enter the park during hunting season.

"The cove is like a fortress. It's protected on three sides by steep cliffs. To get in, you need to go through a natural tunnel system that's protected by a dog and a sensor. Ric said you'd need a Navy SEAL team to get in there. I said that I didn't know any Navy SEALs, but I did know Mandy-Ray Cruickshank," said Psihoyos.

Cruickshank is one of the world's top freedivers, able to dive 300 feet and come back up under her own power, capable of holding her breath for six minutes. She and freediver trainer Kirk Krack joined the team, swimming into the cove at night to install and retrieve cameras.

Some of the cameras came from Kerner Optical, a spinoff of legendary special effects shop Industrial Light and magic. They build high-definition video cameras into rocks so realistic "that when we went back to retrieve the rocks, we had trouble finding them again," said Psihos.

An electronics expert from the Canadian Air Force built a fleet of unmanned, remote-controlled drones to carry cameras above the hunt. Military-grade heat-sensing cameras were used to track the movements of guards.

The cameras were so cutting-edge that manufacturer Sony hadn't yet released the software necessary to pull data off the hard drives and edit it. The team hid the drives in a hotel air conditioning duct, and within a day of retrieving each one had runners take them to Tokyo or Osaka and send them out of the country.

In the end, only 90 seconds or so of the slaughter itself is shown in the film.

"For me, the most telling violence comes when the violence is over," said Psihoyos. "When the dolphins die, their bodies sink. Divers are sent to retrieve them. Afterwards you see them smoking by the campfire with a blood-red sea behind them. There's something so banal about it. They just had a hand in that atrocity, and life goes on. That's the most revealing part."

The Cove is now in wide release in the United States. The next dolphin hunting season will begin in Japan in September.

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Images: 1) The Cove. 2) Louie Psihoyos.

Brandon Keim's Twitter stream and reportorial outtakes, Wired Science on Twitter.