Alcatraz, known as the Rock, was once among America’s most fearsome prisons, cut off from the free world on a windswept island in San Francisco Bay. Today it is a national park, visited by 1.4m tourists a year, who amble around the famous cellblocks and take selfies against the bars. Until October, if they venture to a derelict building on the island’s north side, they will also encounter giant images of serving and former prisoners.