When underground water gets heated in the deeper end of the earth and come back up, and if they are more than 5 degrees higher than the average temperature of the region, then we have what is called a " hotspring ".

3 reasons for their formation

1. water presence underground:if there is to be water underground, the rocks must be multi-holed, and the temperature simply depends on how deep the water runs. Usually it increases about 30℃for every kilometers underground. This is usually referred to as the "ground-ladder", but if there are 5 to 10 kilometers of cold "ground-ladders", then it means that perhaps the temperature increase below that point may be up to three times normal temperature increase.
2. there must be the still-water effect:there are a number of reasons why still-water effect occurs, but normally it's because of the different densities between hot and cold water, and the result is difference in pressure, which will in turn "push" the underground water up, turning them into hotspring.
3. rocks must have long, deep cracks for hot water to flow upwards in: this is an important passage for the hotspring.

Thus are the three main reasons.