Mike Litterst stood in the cavernous space under the Lincoln Memorial and pointed out that 40 feet up you could see concrete beams that support the 175-ton marble sculpture of Abraham Lincoln.

Visitors in blue hard hats craned their necks to see.

Litterst, of the National Park Service, was leading an unusual tour. Not of the hallowed edifice overlooking the National Mall, but of its vast “undercroft,” which for more than 100 years has been its dark basement, marked by stalactites and old graffiti.

Part of that space is now halfway through a multimillion-dollar project designed to transform it into an expansive visitor area with a museum, theater, store and exhibit section.

The undertaking is believed to be the most extensive project of its kind at the memorial since it was dedicated in 1922, the Park Service said.

It envisions six floor-to-ceiling glass walls that will provide views of the cathedral-like interior of the undercroft, and an immersive theater presentation that will project images of historic events onto screens and the undercroft’s pillars, the Park Service said.

“It’s going to set this town on its ear,” Litterst said last week. “It’s going to be the thing that everybody wants to see. We’re really, really excited.”

The Lincoln Memorial sits well above ground level at the west end of the Mall, and was designed to create the image of “a temple on a hill,” Litterst said. It was modeled on the Parthenon, the Greek temple to the goddess Athena.

The 38,000-ton structure was built on a platform supported by 122 concrete pillars that were sunk through the soft earth to the solid rock below. Dirt was packed around that foundation, leaving about 50,000 square feet of empty undercroft beneath the memorial.

The undercroft is so large that the entire memorial, flipped upside down, would fit in it, said Sam Meyerhoff, senior project manager with the Consigli construction company, which is doing much of the project work.

The effort to use the space was launched in 2016 with an $18.5 million donation from billionaire philanthropist David Rubenstein. But it did not officially get underway until last year, the Park Service has said.

The work “proved a greater challenge than originally anticipated,” Litterst said in an email. “Most of the delay was taken up by the design process, figuring out how to construct the facility without damaging the iconic memorial.”

Some older elements were removed. New ones were added. Several thousand yards of concrete and 400 tons of galvanized metal rebar were used in new structures, said Adam Cirigliano, general superintendent with the Consigli company.

During a test on the old pillars, workers found groundwater only six feet below the surface, he said.

The new area occupies only about 15,000 square feet of the 50,000-square-foot undercroft, Meyerhoff said.

The glass walls are being assembled with special British-made glass that will be heated once in place to keep condensation from obstructing the views, she said.

The project is due to be completed by 2026, Litterst said during the tour last week for representatives of the National Park Foundation. The nonprofit foundation raised $43 million for the project, part of which was Rubenstein’s donation.

The Park Service and the federal government gave another $26 million, Litterst said.

“This is something the Park Service has been thinking about for 50 years,” Will Shafroth, the foundation’s president, said in a video interview Monday. “The power of the public/private partnership is what kept this thing alive.”

“It’s not only going to tell a fuller story of Lincoln, but it’s going to tell a full story of how the Lincoln Memorial has become a part of our nation’s history,” he said.

The memorial was built on a spot called Kidwell Flats, an insect-ridden tract on the Potomac River that had been reclaimed with mud dredged from the bottom of the river.

Ground was broken in 1914. The statue of the seated Lincoln was made from 28 blocks of Georgia marble carved by sculptor Daniel Chester French in the New York studio of the Piccirilli brothers, a team of renowned Italian stonecutters.

The blocks were then shipped to Washington and assembled inside the memorial in 1919.

The memorial was dedicated in 1922 in the presence of Lincoln’s son, Robert, then 78. About 50,000 people attended. African Americans, as was the cruel custom in then-segregated Washington, were shunted off to the rear.

On Easter Sunday in 1939, the African American opera star Marian Anderson elevated the significance of the memorial when she sang there after being barred from performing at Whites-only Constitution Hall, seven blocks away.

Twenty-four years after Anderson’s performance, the Rev. Martin Luther King Jr. gave his “I Have a Dream” speech there before 250,000. The speech’s powerful legacy added another renowned leader to the memorial’s story.

And 50 years later, President Barack Obama, the country’s first Black president, spoke there on the anniversary of King’s speech.

Hundreds of other gatherings to protest, celebrate, pray, mourn and entertain have unfolded against the backdrop of the memorial over the past century.

The cornerstone of the Lincoln Memorial was put in place on Feb. 12, 1915, the birthday of the assassinated 16th president, who led the country through the Civil War and helped put an end to slavery in the United States.

Inside the cornerstone is a sealed copper box that contains copies of the Bible and the U.S. Constitution, a signature of Lincoln’s, a map of the Gettysburg battlefield, a dollar bill and $2.06 in change, a copy of the Feb. 12, 1915, Washington Post and a small silk American flag.

During archaeological work in the undercroft in 1984, Park Service experts found deteriorated boots, bits of clothing, a worker’s cap hanging from a nail, stone carvers’ tools and graffiti.

One piece of the graffiti bears an ornate signature, “Bosco Johnny.”

Litterst said the graffiti is in a remote area of the undercroft not visible to the public, but copies will be on display in various parts of the new space.

On a blustery, sunny day last week, as tourists posed for pictures in front of Lincoln’s statue and signs urged visitors to be quiet, Cirigliano stood outside the memorial and reflected on the project.

“Every single morning, you come out, you’re talking with the guys … you see the sun come up,” he said. “It’s just the best.”