Umi Jigoku (lit. sea hell) is one of the boiling springs that came into existence about 1200 years ago following the eruption of Mt. Tsurumidake. The name was given because its surface is cobalt blue like the sea. Its temperature is about 98°C (208°F), and it is said to be more than 200 m (656 ft) deep. In the park, Amazon royal water lilies and tropical water lilies are grown using hot spring heat, and the best time to see them is from early May to late November.