There are not a lot of people that know about anime and do not know about the Naruto franchise as it is one of the most viewed and highest-grossing anime of all time. This masterpiece by manga author Masashi Kishimoto is considered one of those anime franchises that created the wave of anime worldwide and brought the Japanese art of animation storytelling to the forefront. It is also a part of The Big Three which refers to three quite long and popular anime, besides Naruto the Big Three include Bleach and One Piece.
The reasons for the Naruto franchise's extreme popularity are its unique concept, amazing set of characters, good story, and solid power system. Talking about the power system followed in the Naruto franchise, it consists of chakra, and various forms of jutsu which makes sense since it is a story of shinobis or ninjas. The franchise also has numerous kinds of power-ups throughout the story, and one of the many power-ups is the Sage Mode. It is a powerful state that the shinobis who have learned it can enter into it causing a significant rise in their power and unlocking new techniques. To learn and master the Sage Mode, a Shinobi must have extreme chakra levels and go to the sage institutions that are the places where the Sage Mode(s) originate from and is taught.
Mount Myōboku
Mount Myōboku is one of the two known places in the Shinobi world where the Sage Mode is taught by the great toads. The place is also known as the Mountain of Bewilderment and the Land of Toads and is one of the three unexplored sage regions in the Naruto franchise. It is the same place where the toads summoned by Jiraiya, Minato, and Naruto live. Mount Myōboku is a beautiful land full of rich and unique vegetation. The highest authority of this region is the Gamamaru also known as the Great Toad Sage, Shima, and Fukasaku while the chief toad of the Mount Myōboku is Gamabunta.
There are only two ways to reach Mount Myōboku one of which is going through month-long travel by secret paths from Konohagakure or the village hidden in the leaves and the other is to have a summoning contract with one of the toads from Mount Myōboku and to let it use a reverse summoning. Since the Sage Mode requires a Shinobi to blend the natural energy with his/her chakra thus creating the Senjutsu chakra which allows the Shinobi to enter the Sage Mode, it is taught by the great toad sages, Shima and Fukasaku how to combine natural energy and the chakra.
To do so, one must achieve a balance between the natural energy and the chakra or the spiritual energy because an imbalance or difference in the portions of either of the required energies can cause an error which would lead to a failure in mastering the Sage Mode. To make learning the Sage Mode easier, the toads have come up with intelligent ways like applying a unique oil that can be rubbed on the body of someone who is learning to master the Sage Mode as the oil can make the person's body easily absorb the natural energy. However, if the person who applies this oil cannot maintain a balance and absorb too much of the natural energy, they would turn into a stone forever.
Ryūchi Cave
Ryūchi Cave is the other known place in the Shinobi world where the Sage Mode is taught. It is a legendary place, one of the three sage regions, and a residence of the snakes. The highest authority or the leader of this region is the sacred White Snake Sage who is an expert in Senjutsu with great knowledge that she can pass down to other people. This is also the place where people like Orochimaru and Kabuto trained to achieve and master the Sage Mode, however, only the latter was able to do so since the former did not have a powerful body which is a pre-requirement for learning the Sage Mode.
Ryūchi Cave is only a couple of days distance away from Konohagakure, and it is commonly known that only those who are willing to sacrifice anything in greed of power come to the Ryūchi Cave. To meet the sacred White Snake Sage, one has to pass three dangerous trials posed by the snakes first. As per the rules, if a candidate fails to pass any of the three trials, the snakes are allowed to devour them and steal their chakra. If the candidate succeeds in getting the natural energy injected by the White Snake Sage and survives, they can use the Sage Mode but if their body fails to endure the natural energy, the White Snake Sage devours them the very next moment.
Shikkotsu Forest
Shikkotsu Forest is also one of the three unexplored sage regions and the residence of the slug Katsuyu whose parts are summoned by former Hokage Tsunade and her disciple Sakura. Shikkotsu Forest is also known as the Dry Bone Forest and all slugs who reside there can use Senjutsu easily. Though Tsunade knew how to use the Slug Sage Mode it is not sure who taught her that whether it was the Great Slug Sage or her grandfather the first Hokage Hashirama Senju. Shikkotsu Forest was first discovered by one of the many founders of Konohagakure Kamui Uchiha while he was on his journey of self-discovery.
The first Hokage Hashirama Senju himself was an expert Sage Mode user, his Sage Mode is one of the most powerful in the series. But it is not explained how did he learn and master his unique Sage Mode though, the fans of the series speculate that he learned the Sage Mode from his gifted ability to absorb the natural energy and from the Shikkotsu forest since the nature of his Sage Mode of wood. Maybe, the fans would get to know about how he got his Sage Mode in the Boruto series which is spin-off and a sequel to the Naruto series.