Hydropellica (full name Hyrdopellica Hydroxi) was a freezing cold twilight world. The planet had green and purple skies and at least six moons. It was covered in frost and was very barren; despite this, it supported a number of plants, including thin trees and creepers. It did support animal life, as the dominant predator, the Gappa (which numbered in the hundreds of thousands) hunted all the other species to extinction. It then drew near to extinction itself because of the lack of available food.
The last member of the species was saved by a Modrakanian conservation team. However, their spacecraft crashed onto the Arctic tundra of prehistoric Earth and the conservation team died. The final Gappa survived on Earth, frozen in the ice. The Tenth Doctor and Martha Jones encountered this Gappa in Snowglobe 7 on Earth in 2099, after the section of the arctic containing the creature was moved. (PROSE: Snowglobe 7)