Polypia 4

Polypia 4 was a small planet on which a human colony was established by 2211, when the Eleventh Doctor took Clara Oswald to see the amazing glowing coral.

History
There they found that the peace was disrupted by a giant two-headed shark creature with arms and legs that routinely appeared to scare the colonists before suddenly disappearing. The Doctor made introductions with the five explorers from Earth that had recently decided to settle on the planet. The two-headed shark had been appearing every night but never seemed to attack. The Doctor suspected it had something to do with the coral and took one of the female researchers (a "poor, frightened colonist person"), aboard the TARDIS. From high above the planet the Doctor presented an aerial view from the TARDIS's open door showing the coral to be a giant brain, an organic creature that was alive and capable of dreaming. The colonists arrival gave the coral nightmares — and the sharks were part of that nightmare and not real, just angry illusions created by the coral stress. Clara suggested that rather than building their base on the living coral which was hurting the coral brain the colonists move across to one of the other more boring but less "alive" islands. Not intending to have hurt the coral brain the colonists willingly agreed to move camp and within a few days they were rewarded by the sight of a happier coral brain — now a mass of glowing colours and with even more new and colourful coral growths. The beginning of a beautiful friendship observed the Doctor as he returned to the TARDIS with Clara. (COMIC: Coral Maze)