Viper Mirage is an anime-exclusive technique used by Viper that unleashes illusions over a large area, changing its appearance. Viper generally uses it by covering up places in their chosen location and making new sections.
Although the area might appear different, it's still all just an illusion, and as such doesn't physically change. A user who remembers the area's appearance can navigate through the illusion by recalling what the area actually looks like.