The Marble Zone is an area of South Island where few dare to venture. It appears to be some form of ancient ruin which sits on a volcanic area of the island. No history has been revealed regarding the Marble Zone but it is littered with all manner of traps, both naturally occurring and artificial. There are pools of boiling lava everywhere, small isles that sink under the slightest weight and large steel crushing presses with spikes that are attached to chains.
The Zone has only fully appeared in once in Mayhem in the Marble Zone. Tufftee Acorn came here after stealing an old pair of Sonic the Hedgehog's Power Sneakers so he could fight against Robotnik, but he was captured and tormented by a gang of Badniks who planned on turning him into a Badnik too. Sonic arrived just in time to save him and destroy the Badniks, though almost got killed by a spiked crusher. Tufftee managed to run in and shove Sonic out of the way and the two of them returned to the Green Hill Zone.
Before this, the area had appeared in Cartoon Concerto, a story in the first of The Official Sonic the Hedgehog Yearbooks. After watching cartoons to get some inspiration, Robotnik tried to use the natural hazards of the Zone to capture Sonic, such as painting a tunnel onto a wall or painting glue next to an active volcano. All of these plans resulted in Robotnik coming out worst.
The Marble Zone was mentioned during the Mock the Doc TV show, as Teddy Gormless, one of the contestants, is the only known resident of the Zone. Elsewhere, Sonic briefly sped through the Zone to stop a Space Satellite in Time Racer and first had to traverse it in flashback story Kintobor spelled backwards is.... Sonic had also just come back from the Marble Zone before Robofox and had brought something back to show Tails, but we Boomers never got to see what.