Throughout the Zamfara region in northwestern Nigeria, children are dying at an alarming rate. What exactly could be causing such an epidemic? The answer lies in the unique geology.

In the Witwatersrand goldfields, not far from bustling Johannesburg, South Africa, more than a century of mining has left the region littered with mounds of waste and underlain by a deep underground network of abandoned mine shafts, which are gradually filling with water. Today, the mines are producing less and less gold and more and more sulfuric acid.