The Floating Island is a location that appears in Sonic Underground. It is an island that floats through the skies of Mobius, suspended by the mystical powers of a Chaos Emerald. It is home to the echidna people and is fraught with danger to keep out intruders, such as various traps and anti-aircraft systems. The island is protected by its guardian Knuckles the Echidna, a vital ally to the resistance.
Years ago, Queen Aleena Hedgehog came to the Floating Island, leaving a message for her children Sonic, Sonia and Manic saying that Knuckles would be the first among their allies for the showdown with Dr. Robotnik.
Sonic, Sonia and Manic came to the Floating Island years later in hopes of finding their mother. At the same time, Sleet and Dingo found Knuckles on the island (who initially took them prisoners) and convinced him that the Sonic Underground was after the island's Chaos Emerald. While Knuckles was busy dealing with the hedgehogs, Sleet and Dingo went after the Emerald themselves for Robotnik so the island would crash and take the hedgehogs with it. Fortunately, Knuckles and the hedgehogs cleared up the misunderstanding in time to get the Chaos Emerald back just as the villains stole it and the island began plummeting. After the villains had fled and the island was saved, the Sonic Underground and Knuckles forged an alliance under Aleena's message.
The Sonic Underground later returned to the Floating Island to ask Knuckles (who initially mistook them for an enemy and tried to shoot them down) for help to take down the Fortress of Altitude. While Knuckles was reluctant to abandon his island, the group convinced him. Before leaving, Knuckles' great-grandfather Athair warned him that the future of Mobius would depend on his choices on his journey.