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Hagia Sophia mosque, Istanbul, Turkey: This soaring cathedral was built in 537 A.D. at Constantinople, today's Istanbul. In 1453, Constantinople fell to the Ottomans and the church became a mosque. Christian mosaics were revealed when the landmark became a museum in 1935.

Notre Dame de Paris cathedral, Paris: During the 19th century, Notre Dame de Paris was in such a state of disrepair that city planners considered tearing it down. Novelist Victor Hugo, an admirer of the French Gothic structure, wrote "The Hunchback of Notre Dame" to help draw attention to the cathedral’s plight. Success of the book sparked renewed interest in the building and led to a fundraising campaign that financed the cathedral’s 1845 restoration.

Red Square, Moscow: Domes with golden cupolas surrounded by red brick walls are at the heart of Moscow's Kremlin. The Cathedral of St. Basil the Blessed on adjacent Red Square features nine towers of different colors

St. Peter’s Basilica, Vatican City, Rome: St. Peter's is one of the most recognizable churches in all of Italy. During the 16th century, Michelangelo worked extensively on the building; in December 2007, a red chalk drawing for the basilica’s dome, Michelangelo’s last known sketch, was discovered in the Vatican archives.

Piccadilly Circus, London: While the name "circus" technically means a circle, the traffic at this five-road intersection in London’s West End can certainly be circuslike. It’s not only a station on the Underground but a busy shopping district – which is fitting, given its name also derives from the success of one particular tailor who made collars called "piccadills" in the 1600s

Colosseum, Rome: The 50,000-seat amphitheater in Rome was inaugurated in 80 A.D. by Emperor Titus, and served as the backdrop for gladiator duels, battle re-creations and dramas. The word "arena" comes from the Latin word for sand, which always covered the colosseum floor.

Stonehenge, Wiltshire, England: How and why this circular monument of massive rocks was created between 3,000 and 1,600 B.C. is unknown, but some experts say the stones were aligned as part of a sun-worshipping culture or astronomical calendar. Recently scientists have determined that the structure also served as a burial ground.

Neuschwanstein Castle, Bavaria, Germany: The inspiration for the Sleeping Beauty Castle at Disneyland, Neuschwanstein is a creation of "Mad King" Ludwig II of Bavaria. Perched on a peak in the Alps, the gray granite castle rises to towers, turrets and pinnacles.

Giants Causeway: This natural phenomenon on Northern Island’s coast, near Bushmills, consists of approximately 40,000 interlocking basalt columns, some more than 20 feet tall, created by an ancient volcanic eruption. The effect looks like steps big enough for a giant to walk into the sea.

The Parthenon on the Acropolis, Athens, Greece: Work began on the temple to the goddess Athena in 447 B.C., and the result was to become one of the world’s most famous examples of Classical Greek architecture. This set-in-stone wonder attracts millions of visitors each year, though restoration efforts are trying to stem the tide of time and pollution on the marble statues and friezes.

Alhambra, Granada, Spain: The palace and citadel — once the residence of the Moorish caliphs who governed southern Spain — feature mosaics, arabesques and mocarabe, or honeycomb work.

Custom House, Dublin, Ireland: The Georgian-style domed Custom House is prominently situated on Dublin’s skyline, and is arguably the most famous building in the city. The 14 keystones over the doors and windows are known as the Riverine Heads because they represent the Atlantic Ocean and the 13 principal rivers of Ireland.