DEEP & BROODY

On the side of a hill next to some thermal springs 3,000m beneath the surface of the Pacific Ocean, scientists have discovered the world's biggest octopus nursery. Join them as they explore it on Planet Earth III

- DR HELEN SCALES

A mauve arm, covered in suckers, gently unfurls and tends to a clutch of eggs shaped like elongated ping pong balls. Puffs of water from the siphon on the side of the octopus's head ensure her unhatched young get plenty of oxygen. She's surrounded by hundreds of other females which, when viewed from a distance, live up to their nickname. Pearl octopuses (Muusoctopus robustus) resemble spherical gems sitting on the seabed.

This is the largest known aggregation of eight-armed molluscs on the planet around 20,000 and it's being witnessed by people all around the world in stunning high definition in the 'Oceans' episode of the BBC series Planet Earth III.

This view would have been astonishing enough had it come from somewhere in the shallow seas, a tropical coral reef or a kelp forest, but these octopus mothers are tending their eggs almost 3km (2 miles) below the surface, in the freezing cold and darkness of the deep sea.

"The fact life is there at all is amazing," says producer and director Will Ridgeon, who spent two years filming the octopuses and collaborating with scientists and engineers at the Monterey Bay Aquarium Research Institute (MBARI) in California.

The Octopus Garden, as the site is now known, is located in the eastern Pacific, 160km (100 miles) southwest of Monterey Bay, on a hillock near a giant underwater mountain called Davidson Seamount.

The site was discovered in 2018 during an expedition that was being live-streamed over the internet. It was the first time anyone had seen so many of these creatures in one place, let alone in the deep sea (octopuses are notoriously solitary animals and when kept together in captivity they tend to become cannibalistic).