Radar and laser scans have revealed hidden military tunnel and fortifications underneath the infamous Alcatraz federal penitentiary.

Fort Alcatraz

The Alcatraz island off the coast of San Francisco is known as a former high-security federal prison that once held the most notorious criminals in the United States. It is now a National Historic Landmark that attracts thousands of tourists who are eager to see what was once known as America's strongest prison.

Unknown to many, the rocky island was also once the home of a U.S. military fortification. This19th-century coastal fortification is known as the Citadel or Fort Alcatraz. It was built in 1859 but was later bulldozed when the penitentiary was built.

Findings of a new study now reveal the remains of the military structures remain underground and are surprisingly in good condition.

Unveiling Buried Military Structures

Using terrestrial laser scans, ground-penetrating radar and a process called georectifications, researchers were able to locate and assess the remains of the historic structures beneath the former recreation yard of the infamous penitentiary.

The georectification process involved linking old digitized maps to a coordinate system so they can be geolocated in 3D space.

"Geophysical interpretations based upon an attribute analysis of ground‐penetrating radar data are combined with terrestrial laser scans, georectifications based on historical documents, maps and photographs to develop an iterative model for locating and determining the extent and integrity of subsurface historical architectural remains beneath the former recreation yard of the Alcatraz penitentiary," archaeologist Timothy de Smet, from Binghamton University, and colleagues wrote in their study.

The nondestructive research method revealed the remains of the buried structures, which include a bombproof earthwork traverse, a tunnel going through a long mound, which runs from east to west underneath the recreation yard.

Remain In Good Condition

de Smet said he was surprised to find that the historical structures are still in good condition given the location and nature of the structures.

"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," de Smet said.

The findings were published in the journal Near Surface Geophysics on Jan. 17.