It seems impossible to think that a city could have an underground rail system that most people don't even know about. But that's the case with these four secret subways hidden beneath the bustling streets of some of America's biggest cities.

1. The Wet & Windy City

Starting in 1899, the Illinois Telephone and Telegraph Company dug under most of downtown Chicago, creating nearly 62 miles of tunnels, six feet wide by seven and a half feet tall. Their original intention was to house telephone cables, but the company also installed tracks to make getting around easier. Spotting an opportunity, they renamed their business The Chicago Tunnel Company in 1906 and became an underground delivery service instead.

At their peak use, the tunnels buzzed with around 150 small locomotives, hauling 3,300 miniature train cars that delivered 600,000 tons of freight every day. Using special elevators connected to the tunnels, businesses like Marshall Field's would get new clothing and shoe shipments from the rail, but delivering coal for furnaces was the company's bread and butter. However, by the late-1940s, most buildings were using natural gas for heat and those still using coal were getting it by truck, which was much cheaper. Business declined until the company went bankrupt, and the tunnels were sealed in 1959. Shortly after, scrap metal thieves cleaned out the tunnels, including steel doors that were meant to close off the passageways that ran under the Chicago River.

The rails were virtually forgotten until 1992, when a pile driver in the Chicago River hit a freight tunnel wall. A small crack eventually became a 20-foot hole, allowing over 100 million gallons of water to flood the tunnels. Many downtown buildings still had basement connections to the railway, so as the water rose underground it flooded these buildings too, ruining stock in storage rooms, shutting down the Chicago Mercantile Exchange and Board of Trade, and shorting out electrical power for blocks. Days later, the hole was repaired and the water was pumped out. The clean-up cost and estimated damage to downtown businesses was more than $1 billion. Since then, many sections of the tunnel have been closed off, while other branches have come full circle—they're once again being used to house telecommunication wires.

2. The Subway You Paid For (And Probably Didn't Know It)

Once called "the shortest and most exclusive railway in the world," the U.S. Capitol Subway—AKA "The Senate Subway"—is a little-known secret to most Americans. Initially built in 1912, a small, two-line monorail system linked the Capitol building to the Russell Senate Office Building just 1/5 of a mile away. The open-air cars held 18 people in wicker seats, took 45 seconds to make a one-way trip, and were known to travel back and forth up to 225 times a day when the Senate was in session.

Over the years, the line has expanded to all the Senate Office Buildings, as well as to the House Office Building, allowing every member of Congress to reach the Capitol with ease. The old cars were upgraded in 1965 with new models that include upholstered seats and windshields, most of which are still in use today. In 1993, the cars on one line were replaced with sleek, fully enclosed cabins, and feature an automatic, driver-less system, which normally performs quite well. However, in May 2009, the train broke down, stranding Senators Voinovich, Lieberman, Alexander and McCaskill between stations. McCaskill let the world know via Twitter that their train had stalled. They were rescued shortly after, but McCaskill was still a little leery, tweeting that it "takes longer, but I think I'll walk."

And in case you were wondering, you don't have to be in Congress to ride the Senate Subway, but you do need special clearance to do so.

3. If You Can't Beat City Hall, Go Under It

New York City's streets were becoming an overcrowded, dangerous place to be. So in 1866, Alfred Beach, scientist, inventor, and publisher of Scientific American, came up with a plan to shuttle people around underground. The concept worked just like the pneumatic tubes for the drive-up teller at your local bank, with a giant fan pushing and pulling the train cars from station to station. But there was one big roadblock to Beach's plan: a powerful, corrupt politician named William "Boss" Tweed. Tweed was accepting bribes and kickbacks from everyone in New York City, including, it's speculated, the businesses that ran private streetcars. Because Tweed had a vested interest in keeping the streetcars going above ground, he fought any proposals to develop public transportation down below. Knowing this, Beach asked for and received permission to build a tunnel for delivering mail via pneumatic tubes.

In a gutsy move, Beach used his permit for the mail tunnel as cover to build a working prototype of his pneumatic subway system. The project was constructed in secret, mostly at night, and cost Beach $350,000 of his own money. When it was finished, the subway featured one velvet-seated wooden train car riding inside a 9-foot diameter brick tube that ran 300 feet down the length of Broadway—right in front of City Hall. The subway started at a lavish station that featured painted frescoes, goldfish swimming in a fountain, and a grand piano to complete the upscale ambiance.

After a grand opening celebration in 1870, thousands visited Beach's subway for a ride. Thanks to public enthusiasm, the state legislature approved funding to start building on a grander scale. But Boss Tweed and the governor aligned to veto the bill, and they succeeded in shutting down the subway a year after it opened. The tunnel was eventually closed and sat forgotten until 1912, when workers adding a new branch to the Brooklyn-Manhattan Transit (BMT) line accidentally broke through the tunnel wall. They found what was left of the station and the wooden car, but it had deteriorated beyond recovery. A few photos of the car were taken before it was destroyed to make room for the new subway line.

4. The Subway That Never Was

During the first part of the 20th Century, Cincinnati was one of the largest cities in the country, with a growth rate nearly the same as Chicago and New York City. And like those cities, Cincinnati had a problem with dangerous, busy streets. So in 1916, a 16-mile mass transit system was proposed to alleviate the congestion. The project included aboveground and underground rails with much of the latter to be constructed by tearing up the Miami and Erie Canal, a man made waterway that had fallen into disuse.

$6,000,000 in bonds were approved in April 1916, but America had entered World War I just eleven days before, and the federal government soon put a freeze on all bond issues. When the war was over, the price of steel and concrete had skyrocketed, so the original $6,000,000 was now insufficient. A modified plan eliminated some of the original 17 stations and cut the track down to six miles, servicing only the western half of the city. With the new plan, construction began in 1920 and lasted until 1925, when the $6,000,000 ran out. During that time, two miles of 26-foot wide subway tunnel were built where the canal had been, and then covered by a new street, Central Parkway, creating a major thoroughfare for aboveground traffic. Until more money could be raised, there were no tracks or train cars, but the infrastructure was in place for the subway's eventual completion.

While city government argued over what to do next, the Stock Market crashed in 1929, World War II stalled the project, and by the 1950s, America was in love with its automobiles, so the demand for mass transit dried up. Today, the tunnel sits, unused and unfinished for nearly 85 years. The entrance to the grand staircase that leads to the tunnel has been closed and most of the aboveground stations have been torn down. There's really very little evidence that the tunnel even exists, which is perfectly fine to some people embarrassed by the project's history.

Over the years there have been numerous attempts to find some use for the tunnel, but none have been successful. Most recently, in 2002, a proposal for mass transit was again considered, but the idea was voted down.

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Does your city have any legends of secret, underground passages? Are there tunnels in your neighborhood you've always wanted to explore? Tell us about them in the comments below.