Warp Ring

Warp Rings are large technological golden rings that when powered by Chaos Energy allows for traveling between Zones in Sonic the Hedgehog's world.

History
After stealing a Chaos Emerald from the Mega City bank, Tails Man opens up a Warp Ring for him and the other Roboticized Masters to escape. Mega Man follows through it, leading to the Green Hill Zone. (Sonic Universe #51)

Dr. Light manages to recreate the Energy, causing it to reopen in the Zone. Mega Man, manages to trick Sonic the Hedgehog (whom he's fighting) in to coming with him through the ring, with both ending up in Mega City. (Sonic the Hedgehog #248)

The two continue their fight, only to call a truce when Tails Man is sent after them. While they're busy, Bass and Metal Sonic arrive at Light Labs to capture Light. Mega Man, Sonic and Tails (cured from Roboticization) arrives too late to stop them. Tails manages to recreate the process needed to open a ring and he, Sonic, Mega Man, Rush and Proto Man enter to find themselves in the Skull Egg Zone. (Mega Man #24)

Later on, Roll manages to open another Warp Ring, allowing Duo to join the others. (Mega Man #27)