Deep inside the Pinguico silver mine in Guanajuato, Mexico, a concrete slab marks where mining operations suddenly came to a halt in 1913 when the Mexican Revolution broke out. The plug was put in place by the Pinguico Mining Company - then listed on the New York Stock Exchange - to safeguard its mine in the hope of a swift return to operations. The departing miners left graffiti on the walls with dates and clues as to who they were, hopefully leaving messages for the future, but Pinguico never came back.