Kuro-Tamago
Kuro-tamago (黒玉子?) — a local specialty of eggs hard-boiled in the hot springs. The boiled eggs turn black and smell slightly sulphuric; consuming the eggs is said to increase longevity. Eating one is said to add seven years to your life. You may eat up to two and a half for up to seventeen and a half years, but eating a whole third is said to be highly unadvised.
-Wikipedia