The valley starts with travertines. It's something between limestone and marble. Thermal springs, rich in carbon dioxide and lime, bubble up to the surface. Once exposed to air, the gas evaporates and the lime settles. Over the years, this sediment layers up, hardens, and turns into porous stone. It’s full of tiny pores and cavities—traces of air bubbles or twigs and leaves that got trapped in the rock and eventually rotted away, leaving hollow spaces.