Demon Island

Sunken Island is Anti-Mobius's version of Mobius Prime's Angel Island. As its name suggests, it is an island sunken under the sea with an air bubble to provide it's occupants with air.

History
the Sunken Island's history is never explained, but it is presumably similar or, more likely, the opposite of Angel Island's history.

The Sunken Island, like it's Mobius Prime counterpart, is also home to the Echidna people, with Anti-Knuckles the Echidna serving as the guardian of what presumably is Anti-Mobius' version of the Master Emerald.