Newport Tower, Newport, RI

Speculation has swirled around this stone structure in Newport’s Touro Park for centuries. Was it an observatory? A windmill? Nobody knows for sure. Twenty-eight-feet tall and estimated to be 500 years old, it’s curiously located and perfectly aligned with other points on the planet, including 5,000-year-old Stonehenge. Some archaeologists even consider the tower to be evidence of pre-Colombian contact in the Americas.

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Church of St. Mary Magdalene, Rennes-le-Château, France

The mystery of this church in southeastern France reaches far beyond Dan Brown novels. Bérenger Saunière, a priest in the village of Rennes-le-Château in the early 1900s, is believed to have discovered the treasure of the Cathars, a Christian sect founded in the 11th century, inside this clue-laden house of worship. The cache of gold is rumored to have been stolen from the Temple of Jerusalem by the Romans in A.D. 70—and possibly connected to the legends surrounding the super-secret Priory of Sion, Knights Templar, Holy Grail, and the

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Porta Magica, Rome

It’s out in the open, but easy to overlook the mystery within Piazza Vittorio in Rome’s Esquilino neighborhood. The Porta Magica (“Magic Door,” a.k.a. “Alchemist’s Door”) is the only remaining entrance to the 17th-century villa of Massimiliano Palombara, who became obsessed with a recipe to turn metal into gold. Given to him by an alchemist, the recipe was written in a secret code that Palombara was unable to read. At a loss, he had the recipe inscribed into the door of his home, hoping that one of The Alchemists of Palazzo

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Coral Castle, Homestead, FL

Made from 1,100 tons of limestone boulders—bigger than those at Stonehenge—this structure, located just south of Miami, was built from 1923 to 1951 by a single man, a tiny Latvian immigrant named Edward Leedskalnin, as his home and an homage to the love of his life who left him the night before their wedding.

Mystery: How did he do it? The jilted man claimed he knew the secret to the pyramids’ construction. Other details—no mortar, precise seams, physics-defying balancing acts—have

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Rubjerg Knude Lighthouse, Hjørring, Denmark

Jutting out from a desolate dune called Lønstrup Klint (cliff), this ghostly sentinel was built in 1900 but abandoned in 1968 after sands and sea began to devour it whole. The sturdy 75-foot-tall building will likely collapse from shifting sands and coastal erosion in the next decade—and it makes you wonder what other ancient Viking relics lie beneath the sand.

Mystery: The tower was built on a dune-less cliff 656 feet from the sea and nearly 200-feet above sea level, yet, despite rescue

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Yaxchilán, Chiapas, Mexico

This obscure fourth-century site, along the Usumacinta River at the Guatemala border, draped in thick strangler vines and echoing with shrieking howler monkeys, is a tourist-free standout among Mexico’s many ruins. Visitors approach by boat, then enter through El Laberinto (The Labyrinth), a limestone building with painted stucco panels and topped with decorative cresteríasdedicated to ruler kings like Moon Skull.

Mystery: Yaxchilán was mysteriously deserted in the ninth century, but

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Skara Brae, Orkney Islands,Scotland

Previously thought to be a Pictish village, this massive and mysterious Orcadian village on the Bay of Skaill is still being excavated—and changing everything we know about Europe’s pre-Celtic era in the process. The 5,000-year-old site predates the Egyptian pyramids.

Mystery: Even though the village was deserted thousands of years ago, the buildings at Skara Brae remain in good condition. Archaeologists don’t know why the last inhabitants left, although

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Renwick Hospital, Roosevelt Island, New York City

This abandoned Smallpox Hospital, replete with granite veneer, corbelled parapets, and mansard roofs, is a reminder of Gotham’s grisly past. Its 100 hospital beds once hosted quarantined immigrants suffering from the gruesome disease. An ongoing $4.5 million restoration project will open Renwick to the public in 2013, kicking off with an art project that includes giant butterflies hovering over the site.

Mystery: Renwick is currently illuminated at night by an anonymous patron, who

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