Researchers at Texas A&M University uncovered some hidden history beneath America's most notorious prison.
The BBC reports scientists have found evidence of a military fortress that was previously thought to have been destroyed. The structures can be traced back to 19th century around the time when gold was discovered in California.
An architect for the Nation Park Service told the BBC, "San Francisco became a very important port for the country and for the west coast, and so protecting it really was the point of building the fortress of Alcatraz."
Not even 20 years later, the American Civil War broke out and Alcatraz was turned into a military installation. This image shows the old structures in red.
But the fortress saw little to no action so it was turned into a prison and destroyed around 1915.
Architects and others involved in the find are still studying the area with radar technology to find out if excavations are necessary.
Alcatraz might have a whole new attraction for tourists to visit.