Halcyon

Halcyon was the capital of the Thran Empire in northeastern Terisiare. It was suspended in the air thanks to powerstone technology. The city hovered above the Caves of the Damned, on which a mana rig (not to be confused with the Mana Rig the Thran built in Shiv) 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. The forces that had been unleashed created the avatar Evra as a silent witness.

The Caves of the Damned would be discovered millennia later by Mishra and Urza, and renamed by the latter as the Caves of Koilos.