The Yucatan is a paradox: a rainforest where rainfall is rare, with no major rivers or streams. Instead, beneath the forest floor lies a mysterious ecosystem, an elaborate web of flooded caves known as ‘cenotes’–Mayan for ‘sacred wells’. More people have walked on the moon than have reached the end of these caverns, but their presence has fueled an astonishing diversity of life: from trees with hundred foot roots that still draw water, to a rich tapestry of exotic animals–like the howler monkeys whose cries fill the lush undergrowth.