10 Largest Megastructures In Star Trek

Long ago, a powerful alien society constructed a ring around their star with a livable interior surface. This surface had highly adjustable environmental settings, allowing it to construct any number of habitats, including Earth-like ones. The so-called Dyson ring was controlled by a sentient artificial intelligence named Vexilon. We were never told exactly how big this ring was, but we have to imagine it would be about the size of Earth's orbit inside the habitable zone of the star, so as to allow liquid water to exist. 

About six million years ago, the species who built the ring ascended into fifth-dimensional energy beings, which left the ring vacant. It was eventually inhabited by a Federation member species called the Corazonians, which is how it got the name Corazonia. 

The Corazonians lived happy, carefree lives creating art and sculptures, and they had a fairly pleasant relationship with their computer overlord Vexilon. Unfortunately, his environmental controls began malfunctioning in 2381, and when the Cerritos arrived to assist with upgrading the ancient AI, they accidentally rebooted his system and caused the environment of the ring to reset to it's default settings, violently transforming it into a primordial landscape before the crew were eventually able to stabalise the system. After its upgraded settings, Corazonia was estimated to remain stable for about another thousand years without issue.