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.

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.

Emergency responders in the Chicago area say they are confident any derailment here could be managed before it reached neighborhood-destroying levels. “Crude is not the threat that everyone says it is,” says Gene Ryan, chief of planning for Cook County’s Department of Homeland Security and Emergency Management. Ryan and a group of first responders looked closely at 29 major accidents across North America and found that “even though the crude is full of all kinds of volatile materials, the cars did not completely blow apart and hit homes,” he says.

But in a city as dense as Chicago, it takes only one freak incident to have a titanic effect on the urban landscape. Just last year, on March 5, on a stretch of track near Galena, Illinois, 21 BNSF Railway train cars carrying 630,000 gallons of Bakken crude derailed and tumbled down an embankment. Five of them burned for three days. At the time, James Joseph, director of the Illinois Emergency Management Agency, told the Chicago Tribune: “We’re fortunate this occurred where it did, in a remote area, and there were no homes around it.”

Experts believe the train was likely headed for Chicago, 160 miles to the east.

Historically, oil in America moved from south (think Texas and Louisiana) to north mostly through pipelines, the safest conduits for it. When newly deployed technologies such as horizontal drilling and hydraulic fracturing—or fracking—opened access to sources of oil in North Dakota and elsewhere in the West, few pipelines were in place to move the crude to the refineries back east that could handle it. (A proposed pipeline for Bakken crude running from Stanley, North Dakota, to Patoka, Illinois, has faced political and jurisdictional challenges.) With limited alternatives, oil producers and refiners turned to railroads. In 2014, trains carried 11 percent of the nation’s crude oil.

So what paths do these tank cars take? The exact routes are state secrets. But assuming 40 trains, carrying three million gallons of crude oil each, pass through the Chicago area weekly, that means more than 17 million gallons roll through the city daily. It’s an inexact count, and the NRDC has continued to push to get accurate information. “A lot of people don’t know their residences are adjacent to hazardous cargo,” says Henderson. “The issue should be subject to public discussion, but the public has been cut off from it.”

Using freight maps and firsthand reporting, the West Coast environmental advocacy group Stand has assembled a national map of the most common crude oil train routes and created an interactive website that allows users to determine how far any U.S. location is from these routes. For example, according to the site, half of Chicago’s Bridgeport neighborhood, home to 32,000 people and U.S. Cellular Field, falls squarely within a half-mile “evacuation zone,” established by the U.S. Department of Transportation for areas vulnerable to crude oil train explosions. Stretch that to the one-mile “impact zone” and you include the Illinois Institute of Technology, University of Illinois at Chicago, and Cook County Juvenile Court.

It’s not just Chicago proper that sees traffic from crude oil trains. They cut through Joliet, Naperville, Barrington, Aurora, and dozens of other suburbs. “I can look outside my office and see them passing through downtown,” says Tom Weisner, Aurora’s mayor. “About 120,000 tanker cars a year now come through our city.”

Last April, the U.S. Department of Transportation ordered a maximum speed for crude oil trains of 40 miles an hour in populous areas. The majority of railroads run them 10 miles slower than that, an acknowledgment, in effect, that the trains aren’t invulnerable. Most often, it is a flawed track, wheel, or axle that leads to a derailment, which can then cause tank cars to rupture.

Bakken crude was first shipped using tank cars designed for nonhazardous materials and ill suited to its volatility. (Most tank cars are owned not by the railroads but by the oil producers and refiners, such as Valero Energy and Phillips 66, that ship crude.) Those first-generation tank cars, called DOT-111s, have almost all been subjected to new protections, including having their shells reinforced with steel a sixteenth of an inch thicker than used in earlier models. Federal regulations passed in 2015 mandate that by 2025 haulers must replace all cars with new models featuring even thicker steel shells and other safety measures.

Railroads know the dangers. In addition to the human and environmental costs, one terrible accident could put a railroad company out of business. Montreal, Maine & Atlantic Railway, which ran the train that devastated Lac-Mégantic, could only cover a fraction of its hundreds of millions of dollars in liabilities and went bankrupt.

The big railroads hauling crude in the United States and Canada have spent heavily on new technology to make their lines safer, including an Association of American Rails app called AskRail, which identifies the contents and location of rail cars carrying hazardous materials. What railroad companies cannot yet do is reroute trains away from the populous areas whose growth their lines once spurred. There simply isn’t the infrastructure in place to do so.

And while the American Association of Railroads reports that rail companies have spent $600 billion since 1980 improving their current routes, even well-maintained tracks remain vulnerable. Department of Transportation accident data shows that broken rails were the main cause of freight derailments from 2001 to 2010. What’s more, the Federal Rail Administration, the agency charged with overseeing the integrity of America’s tracks, says it can only monitor less than 1 percent of the federally regulated rail system annually due to a shortage of manpower.

“There’s a lackadaisical attitude among people, including officials, about infrastructure that is not up to the threats against it, even as the threats are manifesting,” says Henderson. “You saw that in Flint, Michigan, and in other places with drinking water. And now with crude oil trains, which deal with very serious materials moving [on a system] not adequate to protect people from mistakes.”