First, the eggs are boiled slowly in a hot spring pond that has a temperature of about 80 degrees Celsius, for about 60 minutes. When raw eggs are boiled in a hot spring pond, the iron in the hot spring pond attaches to the porous eggshells. Hydrogen sulfide reacts with the iron to produce iron sulfide (which is black), thus resulting in boiled eggs with black eggshells. Then these black boiled eggs are then moved to a steaming pot, in which they are steamed using steam of about 100 degrees Celsius for about 15 minutes. Now the Kuro-tamago are done. As they are also sold at other souvenir shops, they are brought down using dedicated ropeway gondolas.