Umi Jigoku, also known as the Sea Hell, is one of the famous hot spring sites in Beppu, Japan. It is part of the “Hells of Beppu,” a collection of eight unique and captivating geothermal hot spring attractions. Umi Jigoku gets its name from the striking cobalt blue color of its waters, which resemble the depths of the ocean. The intense blue hue is a result of the high concentration of minerals in the hot spring. The water temperature in Umi Jigoku reaches around 98 degrees Celsius (208 degrees Fahrenheit), making it too hot for bathing or direct contact.