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I had believed such a catastrophe was all but impossible. The natural gas industry has many troubles, including chronic leaks that release millions of tons of methane into the atmosphere, but I had thought that pressure regulation was a solved problem. Even if someone turned the wrong valve, failsafe mechanisms would protect the public. Evidently not. (I am not an expert on natural gas. While working on my book Infrastructure, I did some research on the industry and the technology, toured a pipeline terminal, and spent a day with a utility crew installing new gas mains in my own neighborhood. The pages of the book that discuss natural gas are online here.)

The hazards of gas service were already well known in the 19th century, when many cities built their first gas distribution systems. Gas in those days was not “natural” gas; it was a product manufactured by roasting coal, or sometimes the tarry residue of petroleum refining, in an atmosphere depleted of oxygen. The result was a mixture of gases, including methane and other hydrocarbons but also a significant amount of carbon monoxide. Because of the CO content, leaks could be deadly even if the gas didn’t catch fire.

Every city needed its own gasworks, because there were no long-distance pipelines. The output of the plant was accumulated in a gasholder, a gigantic tank that confined the gas at low pressure—less than one pound per square inch above atmospheric pressure (a unit of measure known as pounds per square inch gauge, or psig). The gas was gently wafted through pipes laid under the street to reach homes at a pressure of 1/4 or 1/2 psig. Overpressure accidents were unlikely because the entire system worked at the same modest pressure. As a matter of fact, the greater risk was underpressure. If the flow of gas was interrupted even briefly, thousands of pilot lights would go out; then, when the flow resumed, unburned toxic gas would seep into homes. Utility companies worked hard to ensure that would never happen.