AKTUN TUYUL CAVE SYSTEM, Mexico (AP) — Rays of sunlight slice through pools of crystal water as clusters of fish cast shadows on the limestone below. Arching over the emerald basin are walls of stalactites dripping down the cavern ceiling, which opens to a dense jungle.

These glowing sinkhole lakes — known as cenotes — are a part of one of Mexico’s natural wonders: A fragile system of an estimated 10,000 subterranean caverns, rivers and lakes that wind almost surreptitiously beneath Mexico’s southern Yucatan peninsula.