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 they hit the air, the gas escapes and the lime settles. Over the years, this sediment layers up, hardens, and turns into porous stone. It's full of tiny holes and cavities—traces of air bubbles or twigs and leaves that got trapped in the rock and eventually rotted away, leaving empty spaces.