SAN FRANCISCO—Archaeologists and researchers from Binghamton University, the Golden Gate Recreation Area, and the Concrete Preservation Institute in Chico confirmed that there are historic buildings and tunnels hidden underneath the prison yard at Alcatraz Island. Historians have long suspected that Alcatraz Prison was built over a Civil War-era military fortification.

A study published online on January 17 in the journal “Near Surface Geophysics” indicated a team of eight archaeologists used ground-penetrating radar and terrestrial laser scans and historic maps and photographs. The team used georectifications, a method that involves taking old, digitized maps and linking them to a coordinate system so that they can be geolocated in 3D space, the researchers noted. They found fully buried structures, ammunition magazines and tunnels.

Historians believe that workers built over these existing structures when the prison was built in the 20th century.

One of the buried structures is a “bombproof” earthwork traverse (a tunnel going through a long mound), running east to west under the recreation yard. According to the radar data, this structure was still in good condition.

“These remains are so well preserved, and so close to the surface,” study author Timothy de Smet, an archaeologist at Binghamton University, said to PBS. “They weren’t erased from the island – they’re right beneath your feet.”