Kukai wanted to learn more about Buddhism in a developed country, China, but it was not easy to go to China from Japan.  In the early 9th century, he made a very long and tough journey to China.  He was thirty-one years old at the time.  Finally, he arrived there and learned about not only Buddhism, but also other technologies.  When he came back to Japan, he brought the knowledge back with him to help the lives of the people.  For example, he used his knowledge to improve Man-no-ike in Kagawa, the largest reservoir in Japan.