Beginning in the 1970s, international aid agencies dug hundreds of thousands of wells in Bangladesh to help people access clean drinking water. The effort curbed diarrheal diseases, but it led to a new problem: arsenic poisoning.
Arsenic occurs naturally in some rocks, including formations throughout the Himalayas. When these rocks weather, the groundwater can become contaminated with arsenic. At high doses, arsenic is lethal. But even small doses can cause cancer and other health problems over time.