The Mystic Cave Zone is a dank, cavernous Zone on Mobius. Supposedly deep underground, the location is a mazy labyrinth of tunnels and dungeons, although at least one side opens out into the sea (possibly near the Oil Ocean Zone. Since it appears after the Hill Top Zone in Sonic the Hedgehog 2, it is possibly that the Mystic Caves are underneath the volcanic region.
The Zone is a supposedly haunted cavern that overlooks the sea. The haunting rumours are not entirely unfounded, either, since Captain Plunder and his Sky Pirate crew used the cave as their hideout, and one of their members IS a ghost. Whilst there, Plunder ensured nobody would bother them by installing security doors and hired Badniks that contained no organic battery. Aside from these, the Zone was already a dangerous site thanks to the meddling of Doctor Robotnik, adding an electrified, spiked staircase at the bottom of a pit (although also included the off switch). Robotnik later returned and set up his Doctor Sun scheme, a chance to get the children of Mobius to "volunteer" themselves to be Badniks. Aside from this, his facade had improved the Zone, adding lights to the walkways and generally making it a slightly nicer place to live - if only to trick the kids further.
Sonic the Hedgehog and Miles "Tails" Prower visited the Zone on two occasions in Sonic the Comic, the first time with Johnny Lightfoot and Porker Lewis to rescue Amy Rose from the Sky Pirates, then arrived by Star Post the second time to find Stripes and other children fooled by Robotnik's cult.