Halcyon

Halcyon was the capital of the Thran Empire. It was suspended in the air thanks to powerstone technology. The city hovered above the Caves of the Damned, on which the Mana Rig was built that produced the powerstones the city needed.

Halcyon was destroyed at the end of the Thran-Phyrexian War, when the Null Sphere was sabotaged so the destructive gasses of the stonechargers (which had been used to kill the armies that were attacking the city) couldn't be stopped. All its inhabitants either died from the gas, or fled to Phyrexia.