Do you describe this as a hot spring or hot springs, where there are some hot water pools in one facility as shown in the following photo?
Strictly speaking it should be "spring" if there is only one water source coming out of the ground, or "springs" if more than one. However, in practice the plural would commonly be used for anything of reasonably large extent, as in your photo, even if we can't see what is happening at the actual source of the water.
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Strictly speaking it should be "spring" if there is only one water source coming out of the ground, or "springs" if more than one. However, in practice the plural would commonly be used for anything of reasonably large extent, as in your photo, even if we can't see what is happening at the actual source of the water.