Entering the Slug Acid River at this moment, Naruto immediately felt the incomparably vast natural energy contained within the acid water of the river, and embedded within this natural energy was a portion of "slug nature."
Just as Mount Myoboku's Toad Oil contained toad nature—where absorbing too much without control would result in instantly petrifying into a toad stone statue—the river water of the Slug Acid River where Naruto now found himself was similarly acidic with slug traits. Once one absorbed an excessive amount of this natural energy containing "slug nature" without being able to control it, it would trigger a backlash, causing the practitioner of sage arts to march toward their own destruction.
Such risks in practicing sage arts might be fatal to the majority of practitioners attempting to master them, but for Naruto, this kind of problem simply did not exist.
Chapter 134