Archaeologists have harnessed radar and laser scanning technology to reveal a long-hidden military tunnel and fortifications beneath Alcatraz.

Prior to becoming the site of the infamous high-security prison, Alcatraz Island was home to a 19th-century coastal fortification, known as the Citadel or Fort Alcatraz. Designed to the protect San Francisco Bay, construction of the island’s defenses began in 1860 and increased during the Civil War. There were 140 artillery pieces on the island at the height of its fortification, according to the National Park Service. However, only a few remnants of the fort are now visible.

To unlock Alcatraz’s hidden history, experts performed high-tech scans of the penitentiary’s former recreation yard. In addition to radar and laser scans, they also took old digitized maps and linked them to a system of coordinates. Their research revealed a “bombproof” earthwork traverse and its underlying vaulted brick masonry tunnel, as well as ventilation ducts, according to a statement.

MYSTERIOUS TUNNELS DISCOVERED IN FLORIDA

Binghamton University Archaeologist Timothy de Smet, who led the research, said that the structures were in incredibly good condition. “I was surprised for several reasons,” said de Smet, in the statement. “The remains of these historical archaeology features were just a few centimeters beneath the surface and they were miraculously and impeccably preserved. The concrete veneer of the Recreation Yard floor is incredibly thin and, in fact, in places sitting directly atop the architecture from the 1860s.”

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