Researchers explored pitch-black pits in Yucatán's labyrinthine underwater caves to unlock the secrets of the rivers that provide drinking water to millions

• The Yucatán Peninsula's underwater caves cover over 900 miles.
• Its cenotes, or sinkholes, are an important drinking water resource for the region.
• Researchers mapped unique communities of microbes in a dozen areas of the cave system.

The Sac Actun cave system on Mexico's Yucatán Peninsula contains many wonders, from a 13,000-year-old skeleton to human artifacts and fossils of giant sloths.