There are hidden buildings and tunnels below Alcatraz's prison yard, study finds

So as not to disturb the land by digging, archaeologists used ground-penetrating radar and terrestrial laser scans in conjunction with historical maps and photographs. Below the prison recreation yard, they found fully buried structures, underground ammunition magazines and tunnels. Historians believe when the prison was built, workers elected to simply build over existing structures, rather than take the time and money to demolish them.