Hot springs are natural springs of ground water that are heated to over 100 degrees F. “Mud pots” (pictured on the right), “paint pots,” and “cauldrons” are also hot springs that simply contain more mud.
In the earth's crust the hot rocks, heated by nearby magma chambers, and the hot spot underneath Yellowstone heat up the groundwater. The heated groundwater makes its way to surface through cracks and fissures in the earth’s crust to form a hot pool of water.
The hot springs in Yellowstone have varying temperatures, but very few of them are actually boiling. Usually, the bubbling water in the hot spring is caused by gasses that are being released up into the spring from fractures in the earth’s crust. In the first video you can see that Morning Glory Hot Spring is not boiling and does not have gas bubbling. However, in the second video you can see that Crested Pool is extremely hot and is bubbling. Often, this bubbling is caused by gasses but it is common for the water in Crested Pool to heat up so much that it actually boils.
