Owakudani is a large volcanic caldera that formed 3,000 years ago when Mount Hakone erupted. That hardly sounds like an ideal tourist destination, but hordes of people visit Owakudani each year in search of the mystical black boiled eggs, locally known as 'Kuro-Tamago.'
These black eggs might look other-worldly, but they're actually just plain chicken eggs. The strange black hue comes from boiling them in the sulphur-rich hot water pools of Owakudani. The sulphur in the water reacts with the eggs' shells, making them black and imparting a sulphur tinged flavour and odour to the cooked egg inside.