Kai Province -- the mountainous homeland of the Takeda clan in what is now Yamanashi Prefecture -- was rich in mineral deposits but poor in arable land. When Takeda Shingen seized power from his father and unified the province, he recognized that gold was his greatest strategic asset. He developed mining operations across the region and used the extracted gold to mint Koshu kin, coins prized for their exceptional purity and standardized weight. These coins circulated widely during the Sengoku period and gave Shingen the financial muscle to field armies against rivals like Uesugi Kenshin and the Hojo clan. The Kurokawa mine was a cornerstone of that wealth. Archaeologists have identified more than thirty mineheads, smelter ruins, and the remains of a mining settlement -- a complete industrial ecosystem carved into a mountainside at nearly 1,400 meters above sea level.

Gold extraction at Kurokawa used every technique available to medieval Japanese miners. Workers panned gold dust from mountain streams, the simplest and oldest method, which had been practiced here since at least the early Kamakura period. As Shingen's engineers pushed deeper, they opened pits and drove tunnels into the rock face. Archaeological excavations have uncovered the tools of this labor: stone mortars for crushing ore, iron chisels for breaking rock, along with ceramics and copper coins that speak to the daily life of a mining community perched high on a remote mountainside. Production reached its peak under Shingen's son, Takeda Katsuyori, before the clan's catastrophic defeat and destruction in 1582.

After the Takeda clan fell, Kai Province passed to Tokugawa Ieyasu, and the mines continued to operate under the Tokugawa shogunate's direct control through the Edo period. The gold of Kurokawa no longer financed independent warlords; it flowed instead into the coffers of a unified government. But the veins were not infinite. References to the Kurokawa mine disappear from historical records around the Genroku era, roughly 1688 to 1703, suggesting the accessible deposits had been exhausted after more than a century of intensive extraction. A companion site, the Nakayama mine on the slopes of Mount Kenashi in Minobu, followed a parallel trajectory -- first developed by the Anayama clan as retainers of the Takeda, then absorbed by the Tokugawa, and finally abandoned by the end of the 17th century. Its 124 identified mineheads and extensive tailing piles cover more than 16 hectares.

In 1906, entrepreneurs attempted to reopen the Kurokawa mine, hoping that modern techniques might succeed where Edo-period miners had given up. The venture lasted just two years before it, too, was abandoned. The mountain had yielded what it intended to give. In 1997, the Kurokawa and Nakayama mine sites were collectively designated a National Historic Site of Japan as the Kai Gold Mine Sites, recognizing their importance as a window into Sengoku-era economics and mining technology. Today, the site is about 60 minutes by car from Enzan Station on the JR Chuo Main Line -- a long, winding drive up into terrain that feels centuries removed from the valley below. The forest has reclaimed the mining town. The smelter foundations sit beneath moss and leaf litter. But the shafts remain, dark openings into a mountain that once bankrolled an empire.