Using radar technology, researchers from Texas A&M University found the remains of military buildings and tunnels dating back to 1850.The fort was built as San Francisco's fortunes boomed with the discovery of gold in California and it proved to be of strategic value at the outbreak of the American Civil War in 1861.  'From 1850 to 1907 was the era of Fortress Alcatraz,' Texas A&M Professor of Geology and Geophysics Mark Everett told BBC News.No shots were ever fired from the Alcatraz fortress, and at the beginning of the 20th century it was repurposed as a federal penitentiary.The fortress buildings were thought to have been destroyed - until now. 'The main prison building was built in about 1915, and we have photographs showing how the rec yard was constructed,' says Professor Everett.'But what we don't really know is what exactly became of the fortifications, what state they are in and what is left of the cultural resources. And although it is not always desirable to excavate, with geophysics we can help people to know what is below the surface without actually disrupting it.'The Texas A&M team is using a radar that penetrates the ground and send signals to a receiver, which reflects the structures under the ground. Scans of the prison yard have revealed a subterranean tunnel system that would have been used to transport people and ammunition, and would have been covered in earth to protect it.'We get signatures that indicate there is not only a tunnel, but magazine buildings too.'Another expert, California State University Chico's Dr Tanya Wattenburg Komas, says parts of the fortifications were earthen and covered over with concrete that would have arrived in barrels from Europe, since the U.S. was not producing its own at that time.It is some of the oldest concrete in existence in the United States.Everett and his team believe the most exciting discovery is lying in wait for them under the parade ground at the southern end of the island.'It is called a caponier, and it is a large structure that juts out into the bay and provides defensive cover. We have seen it in the old photographs but it has completely disappeared from present view,' Everett told BBC News.The radar shows a very large structure exists beneath the ground. Archaeologists hope to begin excavating the area soon. Alcatraz Federal Penitentiary was home to the 'worst of the worst.' During its 29-year life as a prison, beginning in 1934, some of the country's most notorious prisoners passed though its cells, including Al Capone and James 'Whitey' Bulger.(dailymail.co.uk)ANN.Az