cenote

A natural sinkhole in limestone bedrock, exposing groundwater beneath — a vertical well into the aquifer, often deep, clear, and blue. Cenotes are characteristic of the Yucatán Peninsula, where they were the primary freshwater source for the ancient Maya and held profound spiritual significance as portals to the underworld. Some are open pools; others are caverns with collapsed roofs; still others are completely underground, accessible only through narrow passages.