Alcatraz, the notorious island prison surrounded by the waters of San Francisco Bay, is slowly giving up its secrets.

Researchers have now discovered extensive tunnels, embankments and other remnants of a military fortress hidden beneath the floors of the prison. Experts had thought these structures were completely destroyed long ago.

The fortress was discovered purely by accident, after concrete experts were called in to repair a small tripping hazard — a hole in the floor of the prison's former recreation yard. When the experts took out a chunk of concrete, it exposed the top of an old battery wall from the 19th century. [Photos: Hidden Fortress Beneath Alcatraz]

Radar reveals lost battlements

Years before it became a federal penitentiary — hosting outlaws like gangsters Al Capone and George "Machine Gun" Kelly — Alcatraz served as a military fortress guarding San Francisco, Oakland and the entire Bay Area.

"We know from records and drawings that Alcatraz, at one time called Fortress Alcatraz, has been heavily fortified," Mark Everett, professor of geology and geophysics at Texas A&M University, said in a statement.

But nobody had any idea that the walls of the 22-acre (8.9 hectare) prison complex sat atop the well-preserved remnants of battlements, tunnels and buildings from the fort, which was established by President Millard Fillmore in 1850.

"At one time, it had 105 cannons that were to be used to protect the Bay Area," Everett said. "We also know that the U.S. Army built underground tunnels and embankments, and we believe we have found the remains of several of these [structures] using ground-penetrating radar."

With ground-penetrating radar, a nondestructive technique, scientists bounce high-frequency radio waves off the ground and measure the reflected signals, which reveal objects or structures buried in the ground.

Everett is part of a team of scientists that includes researchers from California State University, Chico and the National Park Service, which now operates Alcatraz Island as a popular tourist attraction and nature sanctuary.

Concrete evidence

The recent discovery of the fort brought many surprises to light, including the fact that some of the old fortress is made of concrete, which was almost unheard of in North America during the mid-1800s.

"Originally, the fortifications were earthen — they are constructed of dirt — but parts of them had concrete over them to reinforce them," Tanya Komas, director of the Concrete Industry Management program at CSU Chico, told the Houston Chronicle.

"The interesting thing is we weren't even making cement in the U.S. at that time. That probably came as cement in barrels from Europe," Komas said. "To find it on the top of a mid-19th century battery is very exciting."

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