Every year, humpback whales gather in a few tropical locations, such as Tonga and Hawan, to compete for mates.
The event is poorly understood, but it appears to be triggered by a single female coming into heat, which can attract up to 30 males. She swims off with the males in hot pursuit, jousting for 'pole position' directly behind her. Her suitors blow bubbles and slam their chins on the surface to intimidate each other. But, as the competition heats up, they start to collide, forcing each another deeper underwater. It's brutal and the males are often injured. They can even die in this spectacular contest.
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The event was filmed from the air, the surface and underwater. The team had to free-dive (breath-holding without tanks) and place themselves deep underwater in front of the fast-moving train of 40-tonne whales.
Underwater cameraman Roger Munns likened the experience to "going the wrong way on the motorway".