In the original script, the Daleks attempted to kill all their slaves by trapping them in the mines, which will be deluged with lava when the bombs explode. See more
One day, I shall come back. Yes, I shall come back. Until then, there must be no regrets, no tears, no anxieties. Just go forward in your beliefs, and prove to me I am not mistaken in mine. Goodbye, Susan - goodbye my dear.