A hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the Earth's crust. There are geothermal hot springs in many locations all over the crust of the earth.The water issuing from a hot spring is heated by geothermal heat, i.e., heat from the Earth's mantle. In general, the temperature of rocks within the earth increases with depth. The rate of temperature increase with depth is known as the geothermal gradient. If water percolates deeply enough into the crust, it will be heated as it comes into contact with hot rocks. The water from hot springs in non-volcanic areas is heated in this manner.
Blood Pond Hot Spring is one of the “hells” (jigoku) of Beppu, Japan — nine spectacular natural hot springs that are more for viewing rather than bathing. The “blood pond hell” features a pond of hot, red water, colored as such by iron in the waters. It’s allegedly the most photogenic of the nine hells.