Answer: Hotspings are formed due to the heat produced by the interior of the earth. The rocks at certain depths are hot enough that the water when percolates down, mixes up and get heated geothermally.
A geothermal gradient refers to the rate at which the temperature increases with the increase in depth. This geothermal gradient differs slightly from place to place and ranges from about 25-30°C/km.
This is how the mammoth hotsprings were formed.