Regula

From Memory Alpha, the free Star Trek reference.

Regula is a planetoid in the Mutara sector within Federation space, a Class D body according to the planetary classification. Regula is orbited by the research outpost Regula I.

According to Captain Spock of the Enterprise, Regula is "essentially a great rock in space".

It was later revealed that the research team developing the Genesis Device, led by Dr. Carol Marcus and Dr. David Marcus, had constructed a giant cave inside the planetoid, filled with various plantlife and water. According to Dr. Carol Marcus, it had taken the Starfleet Corps of Engineers 10 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)