Kristie Lu Stout speaks to photographer Sim Chi Yin about migrant workers who live underground in Beijing, <script type='text/javascript' src='http://i.cdn.turner.com/cnn/van/resources/scripts/van-widgets.js'></script> <div data-cnnvansinglewidget data-affiliate='gantdaily' data-videoid='tv/2015/02/17/exp-ns-china-rat-tribe.cnn' data-size='auto' data-autostart='true'></div>