The local delicacy of the Japanese mountain town, Hakone, includes chicken eggs that are boiled in sulfuric hot springs and are said to have mythic abilities. According to legend, eating a single Owakudani black egg (kuro tamago) can have some life-altering healing effects.

The jizo statue in the hot springs is said to serve as a "good luck symbol for extending lifespans," so the legend states that each kuro tamago adds seven years to the life of the person that consumes it. To prevent scammers from selling inauthentic black eggs, true kuro tamago are only sold at the Tamago Tea Shop, Owakudani Kurotamago-kan, Yu land, and the Gokuraku Tea Shop.

The hue of the kuro tamago comes from iron sulfide, which is "a result of a reaction with hydrogen sulfide" after raw eggs are boiled in the valley's hot springs, according to Owakudani vendors. The eggs smell and taste a bit sulfuric, but the minerals in the springs may give them even more depth of flavor.