Beneath the Lincoln Memorial is one of Washington's best-kept secrets: the Undercroft, a soaring 50,000-square-foot foundation built to keep the landmark from sinking into D.C.'s swampy grou...

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 in...