In a  speech last year, President Obama used the term "modern slavery" to describe human trafficking. The trafficking of people – for sex or forced labor – is a multibillion-dollar worldwide criminal enterprise that is well organized and operates mostly undercover.       Phillip Martin, senior investigative reporter for our partner WGBH, has been examining human trafficking and efforts to stop the practice. His reporting has taken him to Vietnam, Thailand, and Dubai. In his new series, "Underground Trade,"  Martin reveals how within the United States, "prostituted women, many of them non-citizens, travel by planes, vans, cars, buses, and trains along the Northeast Corridor from New York and back again – a route that could be likened to a reverse underground railroad where individuals, rather than heading toward freedom, are bound in captivity." "Underground Trade" was produced by WGBH in collaboration with the Schuster Institute for Investigative Journalism at Brandeis University and the International Center for Journalists.