A decade on from the National Park Service (NPS) announcing plans to transform the cavernous space beneath the Lincoln Memorial into a museum, tickets for the newest attraction on the National Mall have gone on sale ahead of its opening on June 25, in time for the 250th anniversary of the Declaration of Independence. Tickets for large tours of the undercroft on opening day are already sold out. The underground space at the monument dedicated to the 16th president of the United States is called the undercroft, typically a type of cellar or storage space with a vaulted ceiling. The undercroft features a grid of concrete columns that has been converted into a 15,000-square-foot exhibition space. It details how the memorial was built and the role the site has played in shaping American perceptions of Abraham Lincoln . The undercroft of the Lincoln Memorial in Washington, D.C. Photo by Drew Angerer/Getty Images.…