Romulan Neutral Zone
Nelvana III was the uninhabited third planet orbiting the star Sigma Nelvana, in the Nelvana planetary system. Its surface was nothing but barren rock, with no native lifeforms. During the 2360s, this system was located within the Romulan Neutral Zone, in attack range of fifteen Federation sectors. (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.
However, when the Enterprise-D arrived in orbit, there were no indications of a base at all. The Federation starship was able to perform several scans of the planet, including for artificially-generated power, subspace restriction, gravimetric perturbation, and virtual interferometry and was also looking for reflected pattern return, EM emissions, microwave enhancement, low level gamma imaging, and background radiation, all with a negative results.