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.
It was later revealed that the research team developing the Genesis Device, led by Doctors Carol and David Marcus, had constructed a giant cave inside the planetoid, filled with various plant life and water. According to Carol Marcus, it had taken the Starfleet Corps of Engineers ten months to dig out the underground structure, while the matrix used to terraform the cave only needed a day to take shape. (Star Trek II: The Wrath of Khan)