The Land of the Sky

"The Land of the Sky is held together by a huge glacier which joins the continents. Beneath the continents lie rivers of magma that spread all over the continent like blood vessels. If the tip of the glacier was punctured by an explosion, the magma would rush to the surface, melting all the ice! The very backbone of our Planet Freedom would be shattered! The rotation of the planet would hurl the continents into outer space, where they would be blown to pieces."

- Knuckles' explanation.

The Land of the Sky (天の大地 Ten no daichi) is one of the two dimensions of Planet Freedom. The other being the Land of Darkness. The dimension is filled with various zones and places, most notably: South Island and the presidential house. Somewhere around the dimension, an entrance to the Land of Darkness can be found.

South Island
South Island is Sonic and Tails' home island, a place where surprisingly, an airplane crashed. That airplane is Sonic and Tails' presumable house.

The Presidential House
The presidential house is a place where the president and his daughter, Sara reside. At that residence, Sonic and Tails are tasked by Robotnik, to save his city in the Land of Darkness, called Robotropolis.

Trivia

 * The island also surprisingly floats, just like Angel Island, which originated in Sonic the Hedgehog 3. But instead, this floating island is held by a glacier.