Cu Chi Tunnels, Vietnam

To get under the surface of a destination, one sometimes has to, well, do just that. Here are 15 of the top underground experiences around the world. Built by the Vietcong near Ho Chi Minh City to help them launch surprise attacks on American forces, these tunnels are among the country’s most popular tourist attractions. The entire underground complex snakes for 75 miles — all the way to Cambodia — and includes meeting rooms, triage centers and kitchens. Dress appropriately; going down into one of the original tunnels can be a muddy experience.

Secret Subway Station, New York

Curved arches, exquisite tiling, skylights — in a New York City subway station? Savvy riders have found that if they stay on the No. 6 train after its final stop in lower Manhattan, they can view the lovely City Hall Station as the train makes its turnaround to go uptown. The Transit Museum also occasionally runs tours.

The Bunker, Greenbrier Resort, West Virginia

A Cold War fallout shelter under one of the nation’s most luxurious resorts? Odd but true. In 1958, the U.S. government built this bunker for members of Congress to live in after a nuclear attack. Declassified today and available for tours since 1995, the bunker had dormitories, stocked medical clinics, power plants, a 25-ton blast door and even a crematorium.

Colosseum, Rome

One of the world’s newest underground experiences — the tour debuted in October 2010 — is a place where no one went willingly two millennia ago: the underground chambers of Rome’sColosseum. It was here, after all, that slaves were kept before being forced to fight for their lives in front of enthusiastic audiences.

Churchill War Rooms, London

This secret command center, used by the British Cabinet during World War II, was set in the cellar of London's Treasury Building. Abandoned after the war, it was rediscovered in the 1970s by historians who were amazed to find everything in it virtually untouched since August 1945. It’s a superb museum today.

Titan Missile Museum, Sahuarita, Ariz.

The only remaining Titan missile silo still has a defanged missile in its launch duct. Visitors to this museum can view it, along with a control room in which everything is mounted on springs to minimize damage should a missile land nearby. If you plan well in advance you can request to bunk here overnight, just feet away from the warhead.

Passage Tombs, Newgrange, Ireland

How did our Neolithic ancestors manage to build these massive tombs, carting 16- ton stones from miles away, in such a way that they would remain watertight and intact for more than 5,000 years? That’s just one of the many mysteries surrounding these marvels of engineering, which are hundreds of years older than Egypt’s pyramids and Stonehenge.

Wieliczka Salt Mine, near Krakow, Poland

A UNESCO World Heritage Site, this nine-story hole in the ground is home to exquisite statues, bas reliefs and full chapels carved out of salt by miners over the centuries. The mine goes down nearly 1,000 feet and includes a massive salt lake at its heart.

The Sewers of Paris

Admittedly smelly tours guide visitors through what may be the most famous sewer system in the world. They were expanded and brought up to then-modern standards during the reign of Napoleon Bonaparte, and Victor Hugo immortalized them in his classic novel “Les Miserables.” Included in the experience are a film and museum detailing the history of water and waste in the City of Light.

Cappadocia Caves, Turkey

Cone rock formations, formed by volcanic eruptions millions of years ago and shaped by wind and rain to form caves and labyrinths, have been inhabited at least since the third century, when early Christians hid here from the Romans. Visitors today stay in cave hotels, eat in cave restaurants and enjoy Cappadocia’s unique “lunar” landscape.

Trans-Alaska Pipeline Visitors Center, Fox, Alaska

Here you don’t actually go underground, but view the pipeline doing so. A small museum tells the epic tale of its construction. The pipeline spans 800 miles, crosses mountains and hundreds of rivers, and had to be built to withstand permafrost. It carries roughly 20 percent of all of the oil produced annually in the United States.

Napoli Sotterranea, Naples, Italy

For nearly 2,000 years, the city of Naples got its water from underground cisterns built by the Greeks. Today, visitors tour — partly by candlelight — just a small section of the 250 miles of tunnels and caverns under the city, exploring impressive ancient Greek and Roman ruins and World War II air-raid shelters.

Sutter Gold Mine, Sacramento, Calif.

Visitors here learn to distinguish the real stuff from fool's gold, watch movies about modern-day mining and the Gold Rush, ride a mining shuttle deep into the earth and pan for gold nuggets, minerals such as quartz and obsidian — even gems, such as amethysts and emeralds — during the entertaining two-hour tour. It’s an experience particularly recommended for families.

Catacombe dei Cappuccini, Palermo, Italy

Italy’s most ghoulish site, this crypt houses thousands of corpses, fully dressed and hung from hooks. The practice began in 1599 when local priests mummified a holy monk and allowed visitors. Soon regular residents of Palermo wanted to be remembered in this fashion. Bodies are arranged by gender and profession; their facial expressions often eerily visible.

Underground Tour, Seattle

An old city lies 8 feet beneath the modern one, thanks in part to the Great Seattle Fire of 1889. You’ll hear the oddball story of how that came to pass on Bill Speidel’s Underground Tour, a riveting, rollicking mobile history lesson that begins and ends in a restored 1890s saloon in historic Pioneer Square.