More than 600 pilot whales have washed ashore New Zealand’s South Island over the past two days. Officials and volunteers are working to return the ones that are still alive back to sea, but many have already died or were euthanized because of their injuries

A mass stranding of this scale is almost unprecedented

On the morning of February 10th, more than 400 pilot whales were discovered on a crescent of land on New Zealand’s South Island called Farewell Spit, according to a Department of Conservation news release. Close to 300 of them had already died.

Volunteers with whale-rescue organization Project Jonah and DoC officials managed to send 100 or so whales back to sea on February 11th, only to have 20 wash ashore again. The remaining 80 joined another nearby pod, and appeared to be safe. However, that second pod then stranded itself on Farewell Spit that evening. The New Zealand Department of Conservation sent out a call for more volunteers, warning that it’s unlikely that they’ll be able to rescue all of the beached whales.

While a few pilot whales become stuck in the shallows and on the shores of New Zealand each year, a mass stranding of this scale is almost unprecedented. About a century ago, 1000 whales stranded on islands off New Zealand’s east coast, while another 450 washed ashore in Auckland in 1985, according to a department of conservation news release.

There haven’t been enough large scale surveys to know exactly how many pilot whales are swimming in the deep waters around New Zealand. But scientists do know that the van-sized creatures tend to live in matriarchal family groups of 20 to as many as 100 individuals.

“It’s just awful, this one will certainly make a dent in the New Zealand pilot whale population,” says Liz Slooten, a professor at the University of Otago in New Zealand who studies marine mammal biology and conservation.

Pathologists are conducting postmortem examinations of the whales to try and understand what happened to make so many wash ashore. Because right now, the cause is still a mystery. Sometimes extreme weather or unusual ocean floor topography can make the whales navigate into water that’s too shallow to leave.

Right now, the cause is still a mystery

Marine mammal strandings have also been linked to offshore oil and gas exploration, which use airguns to blast the ocean floor with sound waves. These can directly injure the delicate hearing that marine animals like whales and dolphins use to navigate, or panic them into surfacing too quickly or swimming into the shallows.

With pilot whales, an injury to just one of them could spell trouble for the entire pod. “Generally there are one or two individuals from that group that are in some kind of trouble, either sick, or dying, or sometimes having trouble giving birth,” Slooten says. “The social organization of pilot whales is so strong that when something like that happens, the rest of the group won’t leave. That’s why generally several dozens, sometimes a couple hundred of whales strand.”

But, she adds, “This is really unusual, this large number of whales.”