A recent stranding on remote New Zealand islands left nearly 500 pilot whales dead. Scientists still don't know for sure why the events, also known as beachings, occur, but they have some ideas.

The whales beached themselves on the Chatham Islands, about 500 miles east of New Zealand's main islands. None of the whales could be refloated and all either died naturally or were euthanized.

Australian wildlife authorities are investigating the deaths of the whales, which were discovered Monday on Tasmania's King Island, off the southeastern coast.

"While they are still alive and in water, there is certainly hope for them, but as time goes on, they become more fatigued and their chance of survival reduces," said a government wildlife official.

Video of the area shows large groups of pilot whales stuck on sandbars, trying fruitlessly to free themselves. An estimated 25 whales have already died.