A century-old secret hidden beneath the feet of former President Abraham Lincoln is opening to the public on Thursday, with a new video from the US Department of the Interior teasing some of the never-before-seen views on the National Mall in Washington, D.C.  Visitors will be able to explore the newly unveiled Lincoln Memorial Undercroft, a sprawling 15,000-square-foot museum located directly beneath the iconic monument, for free with advance reservations. For 100 years, the cavernous