Looking at Diana's Punchbowl in the middle of the Monitor Valley, Nevada.  It consists of a large travertine dome that rises about 75 feet above the surrounding plain with a 50 foot wide gaping hole at the top.  The geothermal spring pond is 138*F & tucked 30 feet within, accessible with rope to one side of the pit without falling in.  I couldn't see how far deep the water is but it has a nice blue tone.  It appears long ago that natural cavities allowed the water to flow out of the base of the hill to the south & east where the grass is noticeably greener.  If there were no exit cavities for the water, the hot spring would continue to seep & bubble out of the rim & rebuild travertine terraces (a prime example of this would be Rabbitkettle Hot Springs in the Northwest Territories of Canada & Homestead Crater, Utah).