Nelvana III was the uninhabited third planet of the Nelvana system. This system was located in the Romulan Neutral Zone. Its surface was nothing but barren rock, with no native lifeforms. (TNG: "The Defector")
In 2366, Nelvana III was the subject of an elaborate ruse perpetrated by the Romulan Star Empire to expose Admiral Alidar Jarok as a traitor and to provoke the Federation into violating the Neutral Zone.
Through deliberate misinformation, Jarok was led to believe that the Romulans had established a cloaked base on Nelvana III in preparation for a preemptive strike against the Federation. Faced with the prospect of war, Jarok defected to the USS Enterprise-D to warn them of the attack. To enhance the deception, the Romulans placed a cloaked probe in orbit of Nelvana III that emitted low-level subspace signals.
Before entering the neutral zone, the Enterprise-D completed a tactical scan analysis of the Nelvana system, which showed the orbit and location of Nelvana III in the system. A long range sensor scan of this planet resulted in negative readings which Data interpreted as no unusual activity. The starship was scanning for artificially-generated power, reflected pattern return, subspace distortion, electromagnetic emissions, gravimetric perturbation, microwave enhancement, background radiation, particulate distribution, virtual interferometry, and low-level gamma imaging.
Later, the Enterprise-D entered the Zone for a more in-depth study of the planet. When the Enterprise-D arrived in orbit, there were no indications of a base at all. The Romulans de-cloaked and revealed the trap, but the Enterprise-D was able to withdraw thanks to hidden Klingon reinforcements. (TNG: "The Defector")