At the northern end of the park is Mammoth Hot Springs. This unbelievably massive formation is caused by mineral deposits of travertine contained in the hot spring water coming up from the ground. There are literally mountains of travertine.
In the early days of the park, people would bathe in the terraces, and leave objects in the water for the travertine to build up on. Now a days, they don’t allow anyone to get into the hot springs themselves, however quite a few hats make it!