Hydra was a planet invaded by the Voord, led by Tarlak. The planet was almost entirely covered by water, save for a single landmass: Predora. The planet had large, technologically advanced settlements, and a network of boats and cargo ships, such as the Hopeful.
The First Doctor, Ian Chesterton, Barbara Wright and Susan Foreman helped the native Hydrans defeat the Voord invaders. (AUDIO: Domain of the Voord)