Sensing Barrier

The Sensing Barrier is a giant, dome-shaped barrier placed over a designated area. The barrier is not visible to the standard eye; when viewed with certain dōjutsu, it can be seen that there is a design of several circles which have the kanji for, and inscribed in them. When linked with a Sensing Water Sphere, all unauthorized passage in and out of the barrier can be detected and pinpointed.

This barrier surrounds Konohagakure and is maintained by the village's barrier team.

Trivia

 * In the anime, the Third Hokage promised not to change the formula of the barrier so Itachi Uchiha may sneak back in from time to time to watch over his younger brother.