Sea Hell or Umijigoku in Japanese is one of Onsen Hells in Beppu. It has a beautiful blue color water. This Onsen spring was made about 1,200 years ago. The large pond with blue color was just like Ocean. There is a large pond that has a beautiful water lilly that bloom from May to November.
There is also foot Onsen to enjoy.
Official web site:http://www.beppu-jigoku.com/images/English0.pdf