The Yabe basin, hugging the Kumumoto-Oita border, has for centuries been a location of extreme agricultural fertility - the soil is rich, and water flows everywhere (disclaimer: my mother-in-law is from Yabe, while my father-in-law is from Tomochi, and the former looked down at the latter in two ways).
The question was how to access this basin from the city. Eventually, a series of stone bridges were constructed to facilitate transportation. In fact, 96% of arched brides in all of Japan is distributed in Kyushu, and about half of them are in Kumamoto. (This apparently is due to the Kumamoto-born founder of Taneyama Masonry, Rinichi Fujiwara, who learned the concept of pi). They still exist. Some are large and some are small, but if you follow the rivers up to Yabe, green-colored signs will navigate you. Our friend Yoshiko Toyama provided a beautiful photograph of one. - William