For more than a century, one of Washington's best-kept secrets lay beneath the Lincoln Memorial: the Undercroft, a soaring 50,000-square-foot foundation built to keep the landmark from sinking into D.C.'s swampy ground. Beginning in June, the public will be able to visit the space, now with a museum tracing the memorial's history, from its construction to its role as a powerful stage for the civil rights movement. Correspondent Faith Salie goes underground.