Mysteries come in many forms: ancient, modern, unsolved, and unexplained. But the world's most mysterious buildings are a physical force to be reckoned with. They've become popularized on websites full of user-generated and editor-curated like Abandoned-places.com, weburbanist.com, and AtlasObscura.com, an exhaustive database of the unusual. "In an age where it sometimes seems like there's nothing left to discover, our site is for people who still believe in exploration," says AtlasObscura.com cofounder Joshua Foer. Our definition of mysterious is broad and varied. Some buildings on our list are being eaten alive by the earth, such as a lava-buried church in the...