What is a beached whale and why do whales beach themselves?

Around 100 whales have beached themselves in New Zealand.

Rescue workers are trying to help them but some of the animals have died.

When whales are beached like this it can be really dangerous for them and, if they can't make it back into deeper waters, they don't survive.

Marine scientists don't have one clear reason to explain why whales beach themselves.

Jenny has been looking into the theories...