Shield of Sand

Whenever Gaara is about to come to harm, a shield of sand will automatically surround and protect him. The sand will react regardless of Gaara's will, even protecting him from self-inflicted injuries. This was due to the One-Tailed Shukaku's desire to survive. The shield is capable of withstanding considerably strong attacks and projectiles, and can be hardened to a point where it's stronger than steel. Gaara can also completely surround himself in the shield to further increase his defensive capabilities. The shield is so strong that even Sasuke's Amaterasu failed to breach it, earning the name.

It can be defeated by avoiding it entirely with extremely high-speed attacks before it can intercept them, such as Rock Lee's high speed taijutsu, or merely overcoming the shield with incredible brute force, like Sasuke's Chidori.

Trivia

 * Despite that Shukaku is the source of this technique, this and othe techniques remain with Gaara even after having it extracted from him. Though it is very likely that the technique is no longer automatic.