Hot Springs are one way to heal the health of Corobo and his army. They can only be dug up by Hardworking Farmers. They can vary in size. The larger the hot spring, the more health points they can restore.
Once a hot spring is dried up, it can be refilled by having Corobo go to bed.
Geysers are a sub of the Hot Spring, though they look and are dug the same way. A geyser, instead of refilling health, removes status effects. They are much more common in places where status effects are common.