Mount Mobius

Mount Mobius is asserted to be the highest mountain on Planet Mobius. This behemoth of rock and vertigo doesn't need a continental shelf to bulk up its altitude; the mountain is a single, titanic volcano, rising up out of the sea and being generally intimidating to nearby observers. As the Veg-O-Fortress, it is the setting for Sonic the Hedgehog Spinball—and at the end of the game, the mountain sinks beneath the waves. It is a tall volcanic mountain covered in ice, the inside turned into a mechanical pinball fortress.

Veg-O-Fortress
What exactly Mt. Mobius looked like in its pristine, natural state is not something we ever get to know, for the mountain makes its first appearance only after it has attracted the attentions of Doctor Robotnik. In the style of evil geniuses everywhere, the good Doctor decides that loftiest peak of Planet Mobius would provide an excellent site for another of his evil bases; and the locale is all the more attractive by the fact that Mt. Mobius' furious volcanism will provide ample electricity for his deranged schemes.

Thus, by the time Sonic and Tails arrive, the towering spire of rock has been mechanized into the Veg-O-Fortress. This industrialized leviathan includes four distinct levels: After pretty much everything inside the Veg-O-Fortress gets wrecked during the events of Sonic the Hedgehog Spinball, Robotnik's artificial facility collapses. The end sequence of the game shows Mt. Mobius and the remnants of the Veg-O-Fortress falling beneath the waves. Consequently, the mountain is destroyed.
 * Toxic Caves: Sonic is flushed into a stinky cavern full of toxic sludge and mine carts, which just so happens to be the place where Robotnik dumps all of the industrial waste.
 * Lava Powerhouse: Sonic ascends to the next level, a power station of sorts that uses magma and steam energy to run various machines.
 * The Machine: At the industrial heart of the Veg-O-Fortress lies an immense, automated Badnik production plant, complete with mass-roboticizer and a central prison for those animals yet to be 'processed'.
 * Showdown: A launch pad at the summit of the mountain, where Robotnik attempts to escape aboard his spaceship.

Adventures of Sonic the Hedgehog
Mount Mobius is also the name of the tallest mountain in the Adventures of Sonic the Hedgehog series. Robotnik used the mind-controlled ape known as Big Daddy to install a laser on top of the mountain, with which he could intimidate and control the people of Mobius. However, Sonic managed to deactivate the laser by draining its batteries. Although the Veg-O-Fortress (which it was called the "Pinball Fortress") appeared in the episode "Attack on the Pinball Fortress".

Sonic the Hedgehog (animated series)
In an early Sonic the Hedgehog bible written by Len Janson, the Roboticizer is referred to as "Veg-o". It is also mentioned there that Bunnie Rabbot was roboticized there. However, the Veg-O-Fortress appeared in the episode "Game Guy" where it was refer to as "The Fortress".