Professor Zaroff was a mad scientist who planned to destroy the world in the 1967 Second Doctor story "The Underwater Menace". Some of his scientific inventions included food made from plankton, and the ability to graft gills to humans to enable them to breathe underwater.
As part of his diabolical plans, he allied himself with the leaders of Atlantis telling them he would raise their city back to the surface or lower the ocean level by draining the water through a fissure in the Earth's crust.
The Doctor immediately realized that this would create super heated steam that could destroy the Earth. Zaroff was defeated when the Doctor and his companions sabotaged the generator he was using to pump the water. Zaroff was left to drown when his laboratory filled with water after the sea walls protecting it collapsed.
He is fondly recalled by Doctor Who fans as one of the most over-the-top villains in the entire history of the show. Particularly well remembered is his cry of "Nothing in the world can stop me now!", which (due to actor Joseph Furst's German accent) was pronounced as "Nuzzing in Ze vurld can ztop me now!" Only two of the four episodes from this story survive, but the surviving material includes that infamous line.