The world is full of man-made wonders -- monuments and structures that, though they seem at first beyond our capabilities,

. There are ancient and medieval wonders like the Great Pyramid of Giza and the Great Wall of China, as well as contemporary ones like the Burj Khalifa, the world's tallest building, which juts a staggering 2,717 feet above the Arabian Peninsula.

Detroit, too, is home to wonders of its own, albeit modest ones in comparison to those of other cities. While we have our share of skyscrapers, our tallest structure, the 727-foot-tall central tower of the Renaissance Center, is only the 107th tallest building in the U.S. But what our wonders lack in grandeur they make up for in quality of engineering and practicability.

The following are the seven man-made wonders of Detroit:

1) International crossings - Detroit's bridges and tunnels

Detroit-Windsor Tunnel

Next time you're stuck in gridlock in the

The nearly mile-long tunnel was completed in 1930 by sealing sections on land and joining them underwater by divers. Over 1.5 million cubic feet of air has to be pumped into the tunnel each

Ambassador Bridge



Whatever your opinion of local billionaire Matty Maroun, you have to admit that the Ambassador Bridge (which he owns) is a Detroit icon. Because of its teal-painted steel and iconic red lettering atop its two towers, the Ambassador is an essential feature of any Detroit video montage (see: "8 Mile"). At the time of its completion in 1929, the 7,490-foot suspension bridge was the longest of its kind in the world. Engineering difficulties confronted the ambitious project almost immediately, as broken wires were discovered in many of its 37 suspension cables.

The Ambassador has stood the test of time, however, and is still the busiest border crossing between the U.S. and Canada. But it will soon have a competitor in the

Michigan Central Railway Tunnel

The Michigan Central Railway tunnel under construction in 1910



In the early 1900s, Detroit was on the cutting edge of technology and engineering, which was perhaps best exemplified in the design and construction of the Michigan Central Railway Tunnel. Prior to its construction, trains carrying goods being traded between the U.S. and Canada had to be disassembled and carried across the Detroit River by barges. A new crossing method was desperately needed to serve the region's burgeoning industry, but there was no precedent for their eventual solution. Upon its completion in 1910, the Michigan Central Railway Tunnel became the world's first underwater tunnel for transporting goods and people.

Check out this

2) The Detroit Salt Mine

Piles of rock salt sitting in front of the shaft of the Detroit Salt Mine





In the far southwest corner of Detroit, across the railroad tracks from Marathon Oil's towering refinery, is an industrial concern whose operations are mostly invisible to the public -- the Detroit salt mine. People have been mining rock salt (also known as halite, the mineral form of sodium chloride) in this location since a shaft was first dug in 1906. In the far southwest corner of Detroit, across the railroad tracks from Marathon Oil's towering refinery, is an industrial concern whose operations are mostly invisible to the public -- the Detroit salt mine. People have been mining rock salt (also known as halite, the mineral form of sodium chloride) in this location since a shaft was first dug in 1906.

According to the Detroit Salt Company, the mine's current owner and operator, the main shaft is nearly 1,200 feet deep (for perspective, that's about twice the height of the Penobscot Building), and the mine itself spreads out across 1,500 acres (or about 2.34 square miles, which is over 1.5 times the area of Belle Isle) below the city of Detroit and its immediate suburbs of Allen Park, Melvindale, and River Rouge. It also contains over 100 miles of underground roads.

According to a 1994 report by the Michigan Department of Environmental Quality, the salt beds beneath Detroit were deposited between 350 and 400 million years ago when a shallow sea that once covered Michigan's Lower Peninsula evaporated. It's estimated that one of the salt beds is over 2,000 feet thick.

Salt has been mined in Michigan since the 1860s, when it was used primarily by the meat packing and leather industries. By the 1880s, Michigan was producing half of all the salt made in the U.S. The Detroit salt mine has been in continuous operation since it opened in 1906 except for a two-year period in the 1980s. Today, the mine's salt is used primarily to de-ice roads in the winter.

So the next time you're in need of some rock salt for de-icing your driveway, consider

Wonder what it looks like 1,200 feet below Detroit in the depth of the salt mines? Check out

3) Detroit River Shipping Channel



"Nature ordained that Detroit should be a great manufacturing city. Its situation on the strait connecting the upper and lower lakes, assuring easy access to the raw materials from forest, soil and mine, its location at the gateway through which east and west passenger and freight traffic would naturally go...all combined to forecast for the place a secure industrial position." -

Nature may have given Detroit a leg up in becoming a great manufacturing city, but it was man's meddling that truly transformed Detroit from a wilderness outpost into an industrial metropolis. This is particularly true of the strait of Detroit, which has been altered significantly over the last 140 years.

The Detroit River has been an active international shipping corridor since the 1820s, serving as the primary means of conveying the bounty of natural resources of the upper Great Lakes to the world beyond. Before the advent of the modern shipping trade, the French settlers who founded Detroit, like the Native Americans before them, made strategic use of the waterway to conduct business.

But as trade along the waterway increased due to the growth of manufacturing in the city and the extraction of natural resources in the northern parts of the state, the need for larger lake-going vessels arose (today the largest ships traversing the river measure over 1,000 feet in length), as did the need for a more for a deeper, straighter shipping channel.

Between 1874 and 1968, various efforts to improve the shipping channel both up- and downriver from the city resulted in the removal of over 46,200,000 cubic meters of material from the riverbed, which is primarily composed of honeycombed limestone bedrock.

Such alterations have proven critical to ships' abilities to transport over 68 million metric tons of commercial cargo that travels down the river annually, but they've also wreaked havoc on natural fish habitats and Great Lakes water levels.

4) Local highway innovations

Detroit is commonly referred to as "the city that put the world on wheels." It also happens to be the first city to put pavement under those wheels. Back in 1909, the stretch of Woodward Avenue between McNichols and Seven Mile roads became the world's first mile of paved concrete highway, a trend that would quickly catch on around the country.

Another highway innovation to occur within Detroit's confines (well, Highland Park's, actually) was the construction of the Davison Expressway, which became the first depressed urban freeway in the U.S. when it opened in 1941.

For better or for worse, our highway innovations have been exported around the world. Now if only we could figure out how to make our roads the envy of the world once again...

5) Relocation of the Gem Theatre



Downtown's Gem Theatre has gone through numerous changes in its 100-plus-year history. At various points it has been a club for wealthy women, a cinema, a theater for plays, and even an adult movie house. But no change was greater than the one it experienced in 1997, when the 2,700 ton structure (that's 5,400,000 lbs) was physically moved five blocks to make way for Comerica Park.

6) Steam system







Some of Detroit's manholes eject plumes of steam that draw comparisons to Gotham City's prodigious output. Above-surface steam may look cool, but it's actually energy loss, often due to faulty equipment and aged piping, much of which was laid in the early 1900s. Some of Detroit's manholes eject plumes of steam that draw comparisons to Gotham City's prodigious output. Above-surface steam may look cool, but it's actually energy loss, often due to faulty equipment and aged piping, much of which was laid in the early 1900s.

Today, properties on the grid serviced by

Yes, the incinerator is controversial. But it's also an ingenious use of resources.

7) Albert Kahn's reinforced concrete factories

Mike Boening Photography via Flickr Creative Commons



One of Detroit's most celebrated architects, Albert Kahn may be best known for art deco masterpieces like the Fisher and Argonaut Buildings, but he was also an incredibly influential industrial architect who made several important contributions to the construction of factories.

Engineers had known since the late 1800s that placing steel in concrete molds distributed the tension that causes cracks and accelerated corrosion. Kahn made several innovations to this method, such as placing the steel beams at angles, that extended the longevity of concrete structures manyfold. This form of construction, dubbed the Kahn System, was used in numerous Kahn-designed industrial buildings, such as the Packard Plant and Fisher Body Plant 21. That's why, despite years of abandonment, scrapping, and exposure to the elements, both buildings remain largely standing and will for decades more.

Kahn was also one of the early adopters of the saw-tooth roof in factories design, which admits light without exposing workers to it directly. Though they declined in popularity in the mid 20th century, Kahn's genius has been proven once again as this style of roofing is back in favor with recognition of the value of natural light in work environments.

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Model D's coverage of STEM (science, technology, engineering, and math) in southeast Michigan is supported in part by the Michigan Science Center. Read other stories in our

Aaron Mondry is a Detroit-based freelance writer. Follow him on Twitter

we somehow find a way to build Detroit-Windsor tunnel , remind yourself of this: you're under water! The engineering required to transport approximately 12,000 subaqueous vehicles across the border every day is remarkable and worth pondering while waiting at customs.minute by 100 foot tall ventilation towers at both ends. Despite the potential for disaster at any second, either from water pressure, structural cracks, or carbon dioxide poisoning, no major problems have arisen in the tunnel's 85-year history -- a testament to the quality of its engineering and construction. And it's still the world's only international underwater automobile crossing. We know that's a lot of qualifiers, but it's still impressive. Gordie Howe International Bridge . Facing staunch opposition by the Maroun family, the state of Michigan and the federal government of Canada, which has pledged to pay over $550 million in construction costs, pursued the project to calm border traffic -- the Howe will be six lanes to the Ambassador's four -- and help flow by connecting to freeways in both countries. While the ultimate design of the new bridge is unknown, preliminary conceptual illustrations are encouraging. Hopefully future video montages will have double the attractive spans. quintessentially '90s video of the tunnel's impressive construction and eventual enlargement.Clarence Monroe Burton, "The City of Detroit, Michigan, 1701-1922"required installing a grid of steel beams underneath the building, a hydraulic system to keep it on a level plane, and 71-inch rubber-tired dollies each the size of a small car. It took nearly a month to complete the farthest relocation ever for a building of its size. Today, the Gem is used exclusively for private events, but the month it traveled was great public theater. Detroit Thermal can opt to heat their building using steam generated from waste at an incinerator in Midtown that's transported through a network of underground pipes. Though many decry the incinerator and its noxious fumes , plenty of energy and environmental experts tout these waste-to-energy plants as a better method of waste management. It's more efficient than a series of individual boilers and makes use of garbage as a resource. @aaronmondry . Matthew Lewis is managing editor of Model D. Follow him on Twitter @matthewjlew