Shizensaigai Is a technique granted to the Mangekyō Sharingan of Tsuna Uchiha's. By utilizing his Mangekyo in a specific manner, Tsuna can cause any natural disaster he desires. Mostly consisting of; Earthquake's, Tsunami's, and etc. Due to chakra channeling, Tsuna can increase the speed, power, and range of the disasters. Due to the death of Tsuna, his Mangekyo was passed down to Sigma, meaning only Sigma currently has access to this technique. Because of Sigma's immense chakra reserves, and extensive gain of the Eternal Mangekyo, Sigma can use this technique as many times as he desires. Currently Sigma has claimed that every time he uses this technique or any technique that Tsuna once used, he see's visions of Tsuna, though these are visions, not much more
Due to the extensive power of this technique, it has been labeled even amongst the Uchiha as a forbidden jutsu.