Snake Way.
At He-El's suggestion, King Kai had turned this million-kilometer-long Snake Way into a trial path for planetary Kamis.
During his travels through the stars, whenever He-El came across suitable planets and members of various races he deemed promising enough to become a planet's Kami, he would toss them into Other World and have King Yemma send them to train on Snake Way.
After passing their training and trials, they would earn the right to reach the end of Snake Way and meet King Kai. Once they passed King Kai's final evaluation and received his appointment, they would become planetary Kamis under King Kai's command.
During He-El's six years of traveling the stars, several Kamis had already been trained and returned to their homelands to take up the role. Under King Kai's observation, the growth potential of humanity on their respective planets had increased considerably, much to his satisfaction.
As all kinds of aliens trained on Snake Way, Goku, Piccolo, Tien Shinhan, Yamcha, Chiaotzu, Krillin, Puar, and the others—who occasionally served as instructors for these aliens—continued training hard for the crisis He-El had warned them about.
Goku and Piccolo, in particular, had received Kami's permission a year after He-El left Earth. Kami brought them into Other World, King Yemma opened a convenient passage for them, and the two competed on Snake Way, racing to Kais Planet to meet King Kai. There, they received King Kai's guidance and trained on Kais Planet.
A step behind them, Tien Shinhan, Yamcha, Chiaotzu, Krillin, Puar, and the others gained Kami's approval in the second year and were allowed onto Snake Way. Arriving at Kais Planet a year later, they trained alongside Goku and Piccolo under the Supreme Kais. Occasionally, while traveling between Earth and Kami's Lookout, they trained while serving as instructors on