SAN FRANCISCO (KPIX) — A new study has confirmed that there are historic hidden buildings and tunnels underneath the prison yard at Alcatraz Island.

Historians have long suspected that when Alcatraz prison was built in the early 20th century, it was constructed over a Civil War-era military fortification.

READ MORE: Smash-And-Grab Thieves Hit San Jose Eastridge Mall Jewelry Store; 5 Sought

Civil War-era building on Alcatraz (National Park Service/GGRA)

The former high‐security federal penitentiary that once housed many of the United States most notorious convicted criminals, Alcatraz Island National Historic Landmark has become one of the most popular tourist attractions in the city.

READ MORE: Investigation Finds Vallejo Officer's Use of Deadly Force Not 'Reasonable'

Instead of digging and disturbing the historic landmark to confirm the theory, archaeologists used ground‐penetrating radar combined with terrestrial laser scans, along with historical maps and pictures.

The study published in Near Surface Geophysics details what they found: fully buried structures, underground ammunition magazines and tunnels that matched the 19th‐century architectural structures depicted in historical documents.

MORE NEWS: San Francisco Lights Up GG Park Christmas Tree After Darkest Year of Pandemic

Archaeologists are now planning more studies to learn additional details about what lies beneath the surface.