In 1992, a leak in a system of long-forgotten freight tunnels underneath the city flooded Chicago's downtown, and caused $3.2 billion in damage.

When Elon Musk got sick of Los Angeles's terrible traffic, he did something about it. He announced his new tunnel-digging company, The Boring Company (TBC), in December 2016.

Originally part of Musk's SpaceX, TBC became an independent company in 2018. The company's first project, using a tunnel boring machine (TBM) named Godot, was a tunnel beneath the Hawthorne, California home of SpaceX that ran more than a kilometre to a parking area.

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When Musk announced a tunnel that would connect the Los Angeles airport, LAX, with Culver City, residents of the wealthy enclave of Brentwood shut the project down. Undeterred, Musk next proposed a tunnel from Hawthorne along Interstate 405 to Westwood.

Then, in June 2018 Musk's company was selected by the City of Chicago to create a tunnel that would run from O'Hare International Airport to downtown Chicago. Downtown Chicago is known as 'the Loop' due to the elevated commuter tracks that encircle it.

The Chicago Flood

Chicago has a long and strange history with tunnels. Monday morning, April 13, 1992, should have been just another day in The Loop, which is home to the city's seat of government, the State of Illinois' seat of government, the Chicago Board of Trade, which is one of the world's oldest futures and options exchanges, and the Chicago Mercantile Exchange, which is a global derivatives marketplace.

For decades, workers in the Loop had heard rumors of a secret tunnel system running beneath the Loop, and it turns out, those rumours were true. Beneath Chicago's streets lies a vast network of freight tunnels that were built during the end of the 19th century and the beginning of the 20th century.

The tunnels were used to deliver coal, merchandise, mail and even money from various banks. By the 1950s though, the freight tunnels were hardly used and were forgotten. Some buildings on the tunnel system blocked up their entrances to the tunnels.

In 1991, a construction company drove wooden pilings into the bed of the Chicago River. Unbeknown to them, they were working right over an old freight tunnel that ran beneath the river.

Chicago freight tunnel 1910, Source: Blankfaze/Wikimedia Commons

Months later, a crew laying television cable in the old tunnel noticed a slow water leak and alerted the city. While the city was researching repair options, the leak was getting bigger and bigger, until at 6am on that April morning, the trickle turned into a flood.

A total of 250 million gallons (1,000,000 cu m) of Chicago River water rushed into the tunnel beneath the river, then spread to the adjoining tunnels. Basements flooded, and workers at Chicago's famed Merchandise Mart reported fish swimming in their basement.

By 8.29am, the lower levels of the Federal Reserve Bank had flooded, and at 12.08pm, the water reached the Chicago Hilton Towers hotel. At the iconic department store Marshall Field's, merchandise stored on the lower levels was drenched.

Freight tunnel under Marshall Field's. Source: Wikimedia Commons

Items stored on the lower levels of Chicago's famed Art Institute were damaged by the water. Some buildings reported 40 feet (12m) of water in their lower levels.

At the Cook County building, workers scrambled to save records stored in their basement that went back more than 100 years. At the State of Illinois Building, tables and chairs in the food court were floating.

Most of the 1,000 restaurants in Chicago's Loop stored their food in their basements, and it was contaminated by the water and had to be thrown away.

The Chicago Board of Trade ceased trading, and Chicago's City Hall was forced to close.

Electrical service to most of the Loop had to be cut, and hundreds of buildings were without power for several days. People living in high rises within the Loop were told to evacuate.

Freight tunnel being built. Source: George W Jackson/Wikimedia Commons

Loop workers who had arrived early that morning couldn't take the subway back home since the power to it had been cut.

On the first day of the Chicago Flood, the Chicago Mercantile Exchange totalled a loss of $25 billion due to not being able to trade.

After 10 hours, workers and the Army Corps of Engineers finally succeeded in plugging the leak and the water drained into a tunnel even deeper than the freight tunnels — the Deep Tunnel.

Freight tunnels intersecting. Source: Wikimedia Commons

Total losses from the Chicago Flood topped out at $1.95 billion in 1992 dollars, which is equivalent to $3.2 billion in 2018 dollars.

The Internal Revenue Service (IRS) offered natural disaster extensions to those living in the Loop who had yet to file their taxes.

In a strange twist, a lawsuit, Jerome B Grubart, Inc. v. Great Lakes Dredge & Dock Co., 513 US 527 (1995), filed against the company that sank the pilings went all the way to the US Supreme Court.

The court ruled that since the work was being done by a vessel in navigable waters of the Chicago River, admiralty law applied, and Great Lakes' liability was greatly limited.

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The six-level 'layer cake' of tunnels beneath the Loop

Besides the freight tunnels, there are five other levels of tunnels beneath Chicago's Loop:

1. The Pedway - closest to the surface is The Pedway, which is designated by its black and gold compass logo plaques that mark the entryways to the underground system.

The Pedway connects the basements of more than 50 buildings in the Loop, including City Hall, the Thompson Center, two Chicago Transit Authority (CTA) stations, a Metra station, shopping centres such as Macy’s, several newer residential buildings and several underground parking garages.

Using the Pedway, you could make the journey from your subway stop, the commuter train station, or your parking garage to your office without ever stepping outside. Numerous businesses make their homes within the Pedway tunnels.

2. CTA Tunnels - beneath the Pedway tunnels, at 20 to 60 feet underground, are the CTA tunnels. Two of the tunnels run beneath the Loop and the Chicago River. The first tunnel runs beneath State Street, and the second tunnel goes under Dearborn Street and Milwaukee Avenue.

Chicago subway tunnels, Source: Chicago Public Library

The tunnels began construction in 1938 where they were part of Franklin Delano Roosevelt’s New Deal and the Works Progress Administration (WPA).

3. Freight Tunnels - underneath the CTA tunnels, and at a depth of 40 feet underground, are the 100km of freight tunnels that flooded in 1992. Built between 1899 and 1906, these tunnels were seven feet six inches (2.3m) high and 6 feet (1.83m) wide and were laid with two foot (610mm) narrow-gauge track.

The freight tunnel system included 19 elevators, 132 30 to 50 horsepower (22 to 37 kW) electric locomotives, 2,042 merchandise cars, 350 excavating cars and 235 coal and ash cars.

The standard freight cars in the tunnel were 12 feet six inches (3.81m) long and 47 inches (119cm) wide, running on two four-wheel trucks, and designed to operate on curves with a 15-foot (4.57-metre) radius.

The underground tunnels followed the street grid above, so you could navigate the freight tunnels using only an ordinary Chicago street map. Air from the tunnels was used to both heat and cool cinemas on the surface since it was a constant 55° F (13° C) year-round.

4. Cable Car Tunnels - between the years 1882 and 1906, public transit before the 'L' subway trains was by cable car. Sitting 60 feet below ground, Chicago’s cable car system was the largest of its kind.

Chicago cable car tunnels, Source: Wikimedia Commons

The cable cars were powered by a single, continuous underground cable, and they ferried commuters to and from the Loop and the city’s North and West Sides. Just a few months after it opened, the North tunnel was used as an escape route during the Great Chicago Fire.

When cable cars fell out of favour, the cable car tunnels were abandoned and sealed.

5. Water Tunnels - in 1867, in order to bring fresh water into the growing city, Chicago built an intake crib 3km out into Lake Michigan. A five-foot tall brick-lined tunnel that was over 10,000 feet long fed that water to a pumping station.

Before 1935, seven more intake cribs were built to feed water to Chicago's two filtration plants. In 1998, one of the water tunnels collapsed, and the city is reluctant to reveal the locations of the remaining tunnels for fear that the information could fall into the wrong hands.

6. The Deep Tunnel - at 350 feet in depth, the Tunnel and Reservoir Plan (TARP) tunnel is where the water from the Chicago Flood drained in 1992.

The Deep Tunnel collects excess rainwater during heavy storms and prevents flooding and pollution. It is one of the largest civil engineering projects ever undertaken. The first phase of the project began in 1975, and it was completed in 2006.

The Deep Tunnel is comprised of a network of 180km of tunnels that can store 2.3 billion gallons of water. Three additional reservoirs will be completed by 2029, and they will be able to store an additional 14.8 billion gallons of water.

This article was written by Marcia Wendorf and is reproduced with kind permission from InterestingEngineering.com. Find the link to the original article here.