Lincoln Memorial's hidden 15,000-square-foot undercroft opens to the public after a century underground

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 U.S. 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 structural chamber has sat vacant, resembling a "two-story shopping mall" supported by a grid of 122 giant concrete pillars.

The project, backed by $26 million in federal funds and $48 million from the National Park Foundation, has been 10 years in the making, with construction officially beginning in 2023, according to Julie Moore, vice president of communications with the Trust for the National Mall.

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Now, a glass-encased exhibit area floats within the infrastructure beneath the statue of the 16th president.

The highly anticipated museum features interactive physical displays, multimedia video presentations and a bookstore.

Among the most exciting historical treasures on display are original copies of the Emancipation Proclamation and the 13th Amendment, complete with Lincoln's signature, according to the Department of the Interior.

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Visitors will also get a unique glimpse into the memorial's construction, featuring historical tools and authentic graffiti left behind by original workers in the 1920s.

Beyond its physical foundations, Moore said, the undercroft tells the story of how the Lincoln Memorial evolved into America's "civic stage."

Exhibits highlight the site's profound role in civil rights history, memorializing historic events like Marian Anderson's 1939 concert and the March on Washington.

Advance reservations can be made through recreation.gov or by calling 877-444-6777.

Visitors can book up to 30 days in advance, or get walk-up tickets on a first-come, first-served basis daily at 8:45 a.m. local time at the nearby Korean War Veterans Memorial kiosk.

Affiliate FOX 5 D.C. contributed to this report.