I n 2009, when C.S. Nautiyal, now the director of Lucknow’s National Botanical Research Institute, spiked a fresh Ganga water sample with an infectious strain of Escherichia coli to test the Ganga’s reported self-healing qualities, he found that the bacteria lasted no longer than three days. He repeated the experiment with a 16-year-old sample of Ganga water—the strain didn’t survive for more than 15 days. Is there something exceptional about the holy river’s water? Such claims are nothing new. In 1896, British bacteriologist Ernest Hankin reported the water’s ability to kill bacteria responsible for cholera. Because of its ability to stay fresh for months, the British always carried water from the Ganga on their ships back to England. And millions of Indians still swear by the water’s mysterious ability to stay clean in their bottles and not smell foul.

There are sceptics, including those who conflate such claims with Hindutva propaganda, but this hasn’t deterred people from researching the...