SMART, Malaysia: This road tunnel in the Malaysian capital Kuala Lumpur also doubles as a storm drain to keep the city safe from flooding.
Aizuddin Saad
Lærdal Tunnel, Norway: Stretching 15.2 miles, the Lærdal is the world's longest road tunnel.
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Guoliang Tunnel, China: Cut into the vertiginous cliff faces of the Taihang Mountains, this tunnel was originally built by locals to make Guoliang village more accessible. Today it's a top tourist destination.
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Large Hadron Collider, France/Switzerland: A donut-shaped tunnel beneath the border between France and Switzerland houses extraordinary research into dark matter.
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Burro Schmidt Tunnel, California: This tunnel in the Mojave Desert is named for the local miner who began work on it to transport ore. It was completed decades later even though a quicker road was built in the meantime.
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Tokyo Bay Aqua-Line, Japan: Also known as the Trans-Tokyo Bay Expressway, this route starts out as a tunnel before emerging via an artificial island to a bridge and causeway.
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Paris Catacombs, France: Originally dug out to provide stones for buildings across Paris, the catacombs were used in the 18th century as a place to store bones from the city's overflowing cemeteries.
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Seikan Tunnel, Japan: This 33-mile tunnel connects Honshu, Japan's largest island, with Hokkaido to the north.
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Glow worm tunnel, Australia: This disused railway tunnel in New South Wales' Wollemi National Park provides a perfect haven for glow worms.
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Road of 52 Tunnels, Italy: Built in just 10 months during the end of World War I, this tunnel was designed to keep mules and men safe from Austrian artillery.
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Greenwich Foot Tunnel, UK: Dug beneath London's River Thames, this foot tunnel once carried workers to busy docklands.
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Eisenhower Tunnel, Colorado: Built at 11,158 feet above sea level, this tunnel -- and its twin, the Johnson Tunnel, cross the Great Divide in the Rocky Mountains.
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Bund Sightseeing Tunnel, China: This levitating train, with added illuminations, provides a link between Shanghai's historic Bund waterfront and the city's modern Pudong district.
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Leake Street Tunnel, UK: This tunnel in south London is home to the largest graffiti wall in London and is a magnet for urban artists.
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Cu Chi Tunnels, Vietnam: Now a tourist attraction, this huge network of tunnels on the outskirts of Ho Chi Minh City was used as a base for Viet Cong attacks on US and South Vietnamese positions during the Vietnam War.
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Drammen Spiral Tunnel, Norway: This road tunnel rises through the Norwegian mountains via six helix loops.
Tomasz Majewski
Channel Tunnel, France/UK: Opened in 1994, this 31-mile tunnel carries passengers and vehicles under the English Channel, creating a fast connection between Britain and mainland Europe.
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Gotthard Base Tunnel, Switzerland: This is the longest and deepest railway tunnel in the world, stretching more than 35 miles beneath the Alps.
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Natural Tunnel, Virginia: Formed over a million years as a result of the limestone and bedrock being dissolved by groundwater, this railway tunnel is now a key attraction in a state park named for it.
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Langjokull Glacier Tunnel, Iceland: Billed as the largest manmade ice structure in the world, the Langjokull Glacier Tunnel winds its way deep into Iceland's second largest glacier, a two-hour drive from the capital Reykjavik.