When free-diver Yang Yun tried to return from the bottom of an arctic pool, she found that her legs had cramped up and she couldn't move.

"I began to choke and sank even lower and I thought that was it for me – I was dead. Until I felt this incredible force under me driving me to the surface."

A Beluga whale named Mila had seen what was happening and sprang into action, guiding Yun safely back to the top of the pool.