The thermal springs carry a mixture of calcium carbonate and carbon dioxide to the surface. As the carbon dioxide de-gasses from the water, the calcium carbonate settles out as a jelly that later hardens into travertine. Breathing in small amounts of this carbon dioxide supposedly cured lung ailments (although the stuff proved deadly in large amounts.) And drinking the waters also helped with digestive issues — it’s basically like drinking bi-carbonate soda (water with Alka Seltzer in it).