A hydrothermal crater
Chinoike Jigoku, or "Blood Pond Hell," is Japan's oldest natural "hell," with written mentions dating back more than 1,300 years. The pond covers an area of approximately 1,300 square meters. It is a hydrothermal eruption crater formed within the Beppu-Shimabara graben, a geologically active zone that runs across the island of Kyushu. The water's surface temperature is a stable 78°C (172.4°F).
The red color comes not from the water itself, which is clear, but from the high concentration of iron oxide and magnesium oxide in the bottom sediment. These minerals are the result of natural chemical reactions occurring under immense subterranean heat and pressure. The steam that rises from the pond is also tinged red by these suspended particles. Geochemical analysis shows the composition of the sediment changes over time. In recent years, an increase in the mineral jarosite has reportedly lightened the pond's color from a deep red to a more yellowish-red. The site is a designated "Place of Scenic Beauty."
Extremophile inhabitants
The hot, acidic water of Blood Pond Jigoku supports a community of extremophilic microorganisms. DNA analysis of the spring's environment has identified a wide diversity of life, including 18 bacterial and 1 archaeal genera. The bacterial population includes known acidophiles from genera such as Acidithiobacillus, Sulfobacillus, and Alicyclobacillus.
Through culture enrichment under low-oxygen conditions, researchers isolated a previously unknown archaeal strain named Sulfolobus sp. GA1. This microbe is a thermophile, thriving in the near-boiling water. Genetic sequencing shows it has a 99.7% 16S rRNA gene sequence identity with Sulfolobus shibatae. The GA1 strain has a metabolism. It readily reduces ferric iron, Fe(III), during its growth cycle. This iron-reducing ability is part of the chemolithotrophic activity that contributes to the pond's mineral composition. The archaeon can also reduce highly toxic hexavalent chromium, Cr(VI), to the less toxic Cr(III). Unlike many related species, Sulfolobus sp. GA1 lacks the ability to oxidize sulfur.