Lethe (The Juggernauts)

Lethe was the planet on which Davros, masquerading as 'Professor Vaso', developed Mechanoids into 'Juggernauts' using organic material cannibalised from human victims. He was unwittingly assisted in the project by Melanie Bush. Lethe initially had an atmosphere which was poisonous to the Daleks, but not Imperial Daleks. It was originally a mining colony. (BFA: The Juggernauts)

Behind the scenes
The name Lethe could be from the mythological river that ran in the Roman/Greek underworld. Supposedly those that drank from it forgot everything.