The Swarm. Jellyfish are beautiful, delicate creatures - and deadly hunters with a ferocious sting. Some are tiny, others the size of small whales.
They fill the world's oceans, but mostly we never see them.
Occasionally, however, they make headlines when they swarm in their millions, invading beaches and closing down power stations. Why do they swarm? And how?
Narrated by David Attenborough. Series producer Mike Beynon