Metebelis III

Metebelis III is a planet with a blue moon that causes the whole planet to have a blue tint at night. It has a wide variety of habitats, such as thick forest, snowy mountains and dry plains. Metebelis III has giant versions of snakes and birds and a sentient species, which the Doctor all encountered on his first trip there in The Green Death.

One of the odd occurrences of Metebelis III is the Blue Crystals. These crystals can focus and amplify telepathic powers and can apparently cause advanced mutations. These Blue Crystals were highly valued by the Metebelis III Spiders, who sent expeditions back in time in order to obtain them.

At some point, the human discovered the planet and landed there. On board the ship were several spiders, which survived the trip and escaped to the mountains. They were then mutated by the Blue Crystals, causing them to grow to enormous size and to develop telepathic powers. The Doctor's encounter with the Metebelis III Spiders and the Great One led to radiation poisoning which led to his regeneration into the Fourth Doctor.