People look at some of the 175 portraits made from Lego pieces in the installation Trace during a preview of the art exhibit @Large: Ai Weiwei on Alcatraz Island Wednesday, Sept. 24, 2014, in San Francisco. Revealing unexpected perspectives on Alcatraz and its layered legacy, the exhibit by the Beijing-based artist prompts visitors to consider the implications of incarceration and the possibilities of art as an act of conscience. The exhibit, which opens Friday and runs through April, features 175 portraits of prisoners of conscience and political exiles around the world, from Nelson Mandela and the Tibetan pop singer Lolo to the American whistle-blower Edward Snowden and composed of 1.2 million Lego pieces. (AP Photo/Eric Risberg)