Zarta (also known as Alpha Z-21) was a class M planet located in the space of the galaxy's Alpha or Beta Quadrants, the homeworld of Krill's people. On Zarta, machines had achieved dominance over the native humanoid population. Because the planet was still a mystery to the Federation by the year 2266, it had earned the nickname Planet Questionmark.
The prosperous humanoid civilization of Zarta began experimenting with machines assuming more roles in society in 2116. Eventually, this led to utopia for the planet until the creation of the city builder, a machine designed to construct entire cities. Soon the machines began directing the planet's resources to themselves, building structures without stopping.
About the time this perpetual building process began, the planet was the subject of a cursory survey by a deep galaxy ship in 2166, using radio film, however the ship was destroyed before returning to its mother ship. Its brief record showed the planet's populace as having a high level of technology, not knowing the dire situation that would develop.
The planet was visited by the USS Enterprise in 2266. The crew discovered that the planet's population was threatened with extinction by the city builders. Captain James T. Kirk and his crew assisted local leader Krill in devising a means to defeat the city builders and reclaim the planet. (TOS comic: "Invasion of the City Builders")