Like any well-adjusted human being with a firm grasp on time management, after seeing Us I spent most of the day reading every single theory on the internet until my eyeballs and brain ached. The movie opens with an eerie bit of trivia about U.S. infrastructure, noting the large number of abandoned tunnels in this country. These tunnels figure heavily into the plot, as they are both a hiding place and prison for the Tethered, a shadow nation of angry clone doppelgängers. They also figure heavily into discussions of plot holes in Us; we never learn how exactly the Tethered, who spent their days pantomiming the actions of their above-ground counterparts, moved around below ground when their counterparts traveled. I have no explanation for international excursions, but the startling vastness of the abandoned tunnels and passageways makes the idea that the Tethered could find their way toward their counterpart simultaneously more and less plausible than it initially seems.

It’s more plausible because of the sheer scale of the U.S.’s subterranean constructions. Some cities have completely overhauled their sewer systems over the decades, leaving behind discarded, musty channels. In Ybor City, a neighborhood in Tampa, Florida, the complex system of brick tunnels running below its streets were rumored to be the work of smugglers, but historians believe they are the vestiges of a 19th-century sanitation system built to accommodate chamber pots and double as storm drains. Most people in Chicago discovered the 60 miles of tunnels running through its downtown in the ’90s only after they flooded; they originally carried small freight trains that would transport coal and packages to office buildings and take away garbage.

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Cities hollowed out many of these underground structures during economic booms and then left them to gather dust once the busts hit, particularly during the Great Depression. Cincinnati has 3 miles of an abandoned subway system beneath its streets and sidewalks because World War I delayed the start of construction and the city gave up on it after the stock market crash, leaving a latticework of tunnels and platforms underground that have never seen the trains they were intended to hold. Rochester, New York, sits above the discarded Rochester Subway, which itself sits atop the original bed for the Erie Canal. New York City also abandoned a number of extensive planned lines during the Depression, including a line meant to run through Bedford-Stuyvesant into downtown Brooklyn. Both the cavernous subway tunnel underneath Boston’s City Hall Plaza and the abandoned trolley passages below Washington, D.C.’s Dupont Circle were decommisioned in the 1960s. Los Angeles also has its own extensive network of abandoned tunnels, including its first subway, the Red Line downtown. Bootleggers and their thirsty customers also carved out passages in DTLA to secretly travel to and from speakeasies, resulting in 11 miles of currently closed booze corridors.

Neglected tunnels aren’t only an urban phenomenon. California is pocked with discarded underground pathways created during the Gold Rush, like the Burro Schmidt tunnel in the Mojave Desert. There are 47,000 abandoned mines in the Golden State alone. Railroad builders contributed their own special twist on the spooky empty passage when their plans were scuttled mid-project or their routes discarded, resulting in deserted structures everywhere from Minnesota to Oregon to Kentucky.

The tunnels in Us, though, are far more elaborate and cleanly than your standard forsaken below-ground tube. They resemble the hallways and classrooms of a stark boarding school, with chalkboards and bunk beds. (Plus bunnies.) While spooky dormitory bunkers aren’t quite as common as transport tunnels, they do exist in the U.S. During the Cold War, the country channeled nuclear anxiety into a new wave of burrowing, resulting in another boom time for creepy subterranean structures. At least 200,000 fallout shelters sprung up (or, rather, down). One such structure: an entire underground elementary school in Artesia, New Mexico, as Will Hunt recounts in his book Underground: A Human History of the Worlds Beneath Our Feet. “The only visible aspect was the playground: Beneath the surface were classrooms for 420 students, and, in the event of a nuclear strike, shelter for 2,000 citizens. The cafeteria’s walk-in refrigerator could be converted to a morgue.” Abo Elementary School is now closed, occasionally used by law enforcement for active-shooter drills.

And so: While it still strains credulity to imagine that millions of people could live their lives below the earth, moving around cities through dirt-covered corridors, it’s not impossible to imagine, because many of those corridors do exist—and at least one does look like a horror-movie school.

The real-life existence of the tunnel systems that lend Us a veneer of possibility also makes it impossible, for a deeply disturbing reason. Downtrodden people do already live in the vast network of unused tunnels in the U.S., especially in major cities.

The 2000 documentary Dark Days—an even more unsettling watch than Us—follows people who created shelters for themselves in the tunnels of Manhattan running from Midtown to Harlem. Its subjects raise pets, bake cornbread, fold clothes; their normality, captured in the literally dark conditions, underlines the way that cast-off structures in cities become dank harbors for the desperate. Amtrak later reopened some of the tunnels featured in the film, forcing the residents from those shelters. While this reduced the number of people living underground, it did not wipe out the practice. Anthony Horton, who wrote the graphic novel Pitch Black about living in the subway system, also died in a fire in a makeshift apartment he had built there in 2012. Most recently, Barstool Sports posted a cruel blog post poking fun at a video of someone seeking refuge in a sewer last year.

This behavior is not limited to New York. It is especially common in Las Vegas, where hundreds of people who cannot afford housing have taken refuge in tunnels beneath the city center originally built to protect from flooding. While these drains minimize the risk of waters rising at street level, they become death traps for the people who live in them when the desert city endures rare rains; in 2016, three people died after they were trapped during a storm. And there have been a variety of incidents across the country where people without homes have dug their own tunnels to live inside, from a single man in New Mexico to groups of people in Olympia, Washington.

While people will probably still be debating the nightmare logic of Us in a decade, the dystopia it presents has an undeniable root in reality. In real life, though, the horrifying thing isn’t people rising from the tunnels—it is that they are down there at all.