In October 2015, a fragile well casing ruptured at the Aliso Canyon natural gas storage field in Los Angeles, California—and no one could figure out how to stop it. For 118 days, 100,000 metric tons of methane and other hazardous pollutants seeped into the atmosphere. The single worst natural gas leak in American history was not only a disaster for the climate; it displaced thousands of nearby residents for months. Even after returning home, many complained of headaches, rashes, nosebleeds, and other symptoms they blamed on the lingering airborne chemicals.

And most of these people didn’t see it coming. The majority of residents near Aliso Canyon claimed they had no idea they lived near a natural gas storage field until the 2015 blowout happened. They didn’t know that if any of the wells ruptured, they were at risk of exposure to a host of toxic chemicals, which could cause serious neurological and respiratory problems and even certain kinds of cancer. They could also be at risk of death from a pipeline explosion, like the victims of the Colorado blast in 2017.

The massive Aliso Canyon storage field, which contained more than 110 underground wells, is just a small part of America’s much larger natural gas infrastructure. Approximately 15,000 such wells are active across the United States, and nearly half of them are concentrated in six states: Pennsylvania, Ohio, West Virginia, Michigan, New York, and California.

For the many thousands of Americans who live near these wells, as well as federal regulators who are tasked with keeping the public safe, these wells are out of sight, out of mind. And a new study shows their dangers to be far greater than previously believed.

Published Monday in the journal Environmental Health, Drew Michanowicz’s study was directly inspired by Aliso Canyon. After that disaster, he and his research team at Harvard’s T. H. Chan School of Public Health wanted to get a better idea of just how many people in America live near similar underground gas storage facilities.

