In this case the crater is not a dip in the ground, it’s a mounded hill. Apparently the water used to bubble up and out of the crater. The hill was formed by the mineral deposits. Some thoughtful person(s) engineered this crater so that the water flows out of it on one side. On the other side there is a tunnel that takes you to the pool that remains inside the crater.
There is still an opening at the top that lets in light and air. There are lights in the water which is why the water glows in the photo. The water is 95° or so and full of minerals. You can pay a fee to float or scuba dive, or do what I did and just take a look.
I saw several other small craters on my morning walks that were posted “do not enter” but I would not be at all surprised to hear that every now and then someone does get into them.
There were small streams on the resort property and the water flowing in them was warm—not at all what you’d expect in a mile-high mountain stream!