Beginning in 1947, Rocketdyne—originally a division of North American Aviation—used the remote hilltops of Santa Susana to test liquid-propellant rocket engines, far enough from Los Angeles that the thunderous test firings wouldn't rattle windows in the Valley. What happened on those hills shaped history. The engines developed here powered the Redstone rocket that launched Alan Shepard into space, the Atlas that carried John Glenn into orbit, and the massive F-1 engines that lifted Apollo astronauts to the moon. For nearly six decades, from 1949 to 2006, the roar of test engines echoed across Ventura County while Angelenos below went about their lives, largely unaware. The facility occupied a plateau in the Santa Susana Mountains, accessed by winding roads through chaparral, deliberately obscured from easy view.

In July 1959, something went wrong inside the Sodium Reactor Experiment, a small nuclear reactor operating in what was called Area IV. Thirteen of the reactor's 43 fuel elements partially melted. Radioactive gases were released. The incident—the first partial meltdown of a commercial nuclear power reactor in the United States—was not publicly disclosed for decades. The U.S. Department of Energy kept it quiet, and it wasn't until investigative reporting in the 1970s and 1980s that the full picture began to emerge. In all, about ten nuclear reactors operated at Santa Susana over the years, along with extensive chemical research. The combination left behind contamination: radioactive materials, solvents, and other hazardous substances that have made cleanup one of the most complex environmental remediation challenges in California's history.

Communities in the surrounding area, particularly the Simi Valley and West Hills neighborhoods, began raising questions in the 1980s and 1990s about cancer rates and environmental contamination. Studies found elevated levels of certain cancers among workers who had been employed at the site. What had been an invisible neighbor became a source of anxiety and anger. By the 2000s, the Boeing Company (which had acquired Rocketdyne), NASA, and the Department of Energy were all parties to cleanup agreements with the state of California. The cleanup has proceeded slowly, complicated by the scale of the contamination, the rugged terrain, and disputes over how clean is clean enough for land that overlooks some of the most densely populated terrain in America.

The test stands are largely silent now. Some structures have been demolished; others stand as rusting monuments to mid-century industrial ambition. The land is technically owned by Boeing, but its future is contested between those who want full environmental remediation and those who argue that complete cleanup is technically impossible and economically prohibitive. A state park has been discussed. Environmental advocates continue to push for accountability. Meanwhile, the dry hills of Santa Susana catch the light each afternoon the same way they always have, giving no outward sign of what lies beneath the soil or what thundered above it during the decades when America was racing to the moon.