Regula was an uninhabited D-class planetoid in the Mutara sector. It consisted of various unremarkable ores. According to Captain Spock of the USS Enterprise, Regula was "essentially a great rock in space". Regula was orbited by the Federation scientific research laboratory Regula I. The Mutara Nebula was located nearby and the Ceti Alpha system was three days away at warp speed.
The Project Genesis team chose this planetoid for their experiments in 2284. The Starfleet Corps of Engineers spent ten months to dig out what would become the Genesis cave. Then, the project initiated the experiment, turning the lifelessness of the planetoid into a new environment, filled with various plant life and water.
Later the next year, when the project team thought that Starfleet was commandeering the project materials, Doctors Carol and David Marcus fled to the planetoid to hide along with the Genesis Device. Admiral James T. Kirk and crew arrived to investigate, and found out that Khan Noonien Singh had tricked them and stole the Device. He tried to maroon them on the planetoid, however, they were rescued by the USS Enterprise which had hid on the other side of Regula. (Star Trek II: The Wrath of Khan)