An anti-gravity engine was an highly advanced technological device used in Ardana's Cloud City of Stratos, which enabled it to remain suspended above the planet over an extended period of decades. Designed by Ardanan Soske Busk, Stratos's anti-gravity engine generated gravitons and anti-gravitons with a component similar to a superconducting cyclotron. The rotational speed of this device controlled the amount of particles created. Control of this engine was through an obelisk-shaped structure called Soske.
In 2377, it was determined that these engines were slowing and threatening to fail altogether, thus resulting in Stratos's crash on the planet. The Starfleet Corps of Engineers team from the USS da Vinci attempted to affect repairs to the engines, but were ultimately unable to prevent the city's descent and destruction. (CoE eBook: Signs from Heaven)