The Scariest Threat You Didn’t Know About They’re explosive. Pervasive. And their movements are cloaked in secrecy. Their nickname? Bomb trains. And they roll through the heart of Chicago.

Above: A train carrying hazardous material, potentially crude oil, heads south through Chinatown’s Ping Tom Memorial Park, photographed at dusk on March 15, 2016. Photo: Jon Lownenstein

They could not look more ominous. The long coal-black tubes announce themselves by their distinctive shape and color, their markings too small to read from the street. The 30,000-gallon tank cars roll, sometimes 100 at a time, in trains of up to one mile in length. Their cargo? Crude oil—as much as three million gallons per train. Nearly all of it is light sweet Bakken crude, a type that is particularly explosive. In whole, these trains constitute likely the biggest, heaviest, and longest combustibles to ever traverse America, and they do so routinely. More pass through Chicago than any other big metro area. Their blast potential has earned them a terrifying nickname: bomb trains.

Stand long enough at 18th and Wentworth, on the traffic bridge that separates the newer sections of Chinatown from the largely residential South Loop, and you will spot the tank cars wending their way across neighborhoods on the Near South and West Sides, past playgrounds, schoolyards, and row after row of houses. An estimated 40 of these trains cut through the metro area weekly. There’s no public information on exact routes or timetables; revealing their paths, the logic goes, might aid potential saboteurs, a real risk in an age of terrorism.

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Until recently, crude on the rails was relatively rare. But since 2008, when Bakken oil began rolling out of newly active fields in the United States—North Dakota is the biggest producer—and toward Eastern refineries, the number of oil tank car shipments has grown 50-fold. That’s pushed the number of accidents up, too. According to U.S. government data, from 1975 to 2012, an average of 25 crude oil spills from tank cars occurred on the rails each year. In 2014, that number rose to 141. Most incidents are minor, such as small leaks. But in cases of a major derailment, the result can be catastrophic, even fatal (see “Terrifying Incidents,” below).

Chicago found that in the last three years there were 17 derailments of crude oil trains in North America significant enough to generate news coverage. In eight of them, the tank cars blew, sending fireballs hundreds of feet into the air, filling the sky with black mushroom clouds. In the most severe cases, the flames produced are so hot that firefighters almost inevitably choose to let them burn out, which can take days, rather than extinguish them. (The Wall Street Journal calculated that a single tank car of sweet crude carries the energy equivalent of two million sticks of dynamite.) Even when there are no explosions, the spills can wreak havoc on the environment: five of the 17 accidents resulted in the pollution of major waterways, affecting thousands of people across the continent.

Chicago is particularly vulnerable. As the Western Hemisphere’s busiest freight hub, the city has become a center for crude oil traffic, too. High volumes, combined with a densely populated urban setting, have watchdogs such as the Natural Resources Defense Council alarmed. Henry Henderson, the NRDC’s Midwest program director, sums up the threat this way: “Trains with highly explosive materials are traveling through the city on aging tracks in cars that are easily punctured, which can result in devastating explosions.”

Many of these trains cut through what were once industrial rail yards in the city and suburbs. Over the last 35 years, however, much of that property has turned into residential and commercial clusters. “You should assume that if you live in the Chicago area, near a railroad track, that there are trains carrying Bakken crude oil,” says Jim Healy, a member of the DuPage County Board.

Though Chicago has so far been spared a crude oil train crash, the potential of one presents a horrifying picture. One particular nightmare is emblazoned in the minds of first responders, and regulators. On July 6, 2013, a runaway crude oil train, which had been left unattended, sped through the center of the small Quebec town of Lac-Mégantic. Sixty-three cars derailed. Forty-seven people were killed, some literally incinerated while they drank at a bar.