The hot springs terraces are one of the most accessible and incredible features of natural wonder in the park. Close to the park's headquarters, you can see them from afar, as they are a patch of white on a dark hill. A boardwalk trail will take you up, down, and close to the springs.
How were they born?
Well... as I said earlier, Yellowstone is in fact living under a magma chamber. When ground water seeps down, it comes in contact with carbon dioxide rising from the chamber. Some of the carbon will dissolve in the hot water and form a mild acidic solution. This mix will dissolve the limestone as it slowly makes its way up through the rock layers as a hot spring. When the steams comes up, the water is released but the limestone becomes solid and makes a deposit forming the terrace travertine we can see today.
Seeing the springs from the first time is quite amazing, especially when one is active (they are not active all the time; when we were there, the activity was quite low and it can remain like that for years). One of the first things you will notice is the smell of rotten eggs so characteristic for hot springs.
The colours, diversity of formations, and eeriness are things to behold. With names such as Palette, New Blue, Minerva, Jupiter, etc., this is a place that is out of this world. We only visited the Lower Terraces, as we had to be home before dark, but the higher terraces can be accessed by a road.
When visiting, please be careful. Do not get off the boardwalk, as you may be scalded by hot water and steam. This is not a joke; some deaths have been reported.
You can get more information about the spring at http://www.nps.gov/yell/tours/mammoth/index.htm