Ballon Terminal is the location where all the lost helium balloons end up, and that is how it stays in the sky. At some point in time, a civilization moved to and thrived in Ballon Terminal, building multiple buildings. Later, this civilization ended by unknown causes, and the buildings fell into ruins.
↑ 3.03.1SBSOne Piece Manga — Vol. 80, fan question: Mr. Oda, heso!! If you look closely, you can see that the sky island known as "Ballon Terminal, the ruined city" has a crapload of balloons stuck to the bottom... but could it be that this sky island is kept in the air with balloons?!