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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.

Colosseum, Rome

One of the world’s newest underground experiences — the tour debuted in October 2010 — is a place where no one went enthusiastically 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 stunned to find everything in it practically intact since August 1945. It’s a superb museum today.

Titan Missile Museum, Sahuarita, Ariz.

The only outstanding 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 reduce spoil should a missile land nearby. If you plan well in advance you can appeal to bunk here overnight, just feet away from the warhead.

Wieliczka Salt Mine, near Krakow, Poland