Mount Rumble, also known as Mount Goro-Goro, is a very tall mountain always surrounded by many clouds and is located in the middle of a mountain range. Mount Goro-Goro and its inhabitants appear exclusively in the Dragon Ball episode "Quicker Than Lightning".
There is a sacred crown on top of Mount Goro-Goro, and if someone grabs it to put it on his head, a thunderstorm will start and a lightning will fall from the clouds and will continuously chase the person who has the crown, trying to hit him/her.
Goku was sent to Mount Goro-Goro by Kami during the young Saiyan's training, to retrieve the sacred crown. Goku climbed the mountain and got the crown, but he ended up being struck by the lightning and stayed unconscious for three days.