A short history of the secret bunker underneath the White House

• President Donald Trump reportedly spent an hour in the White House's underground bunker as early Black Lives Matter protests grew intense in Washington, DC.
• The bunker, known as the Presidential Emergency Operations Center, was built for protection during World War II and most notably used during the 9/11 attacks.
• The terror attacks prompted national security officials to have a more secure bunker built in secret.

When the Black Lives Matter protests following George Floyd's death erupted in America's capital city on May 31, President Donald Trump was holed up in an underground bunker below the White House.

While Trump told a radio interviewer he was there for an "inspection," several sources told CNN's Jamie Ehrlich he was whisked away to the bunker for an hour as the protests grew more intense in Washington, DC. First lady Melania Trump and son Barron were also taken to the bunker.

The White House bunker, formally known as the Presidential Emergency Operations Center (PEOC), is beneath the White House's East Wing. Here's what we know about it.

The PEOC was built as an emergency center

The PEOC was built in the early 1940s during World War II for President Franklin D. Roosevelt. Douglas Brinkley, CNN's presidential historian, called the bunker an "absolute necessity" at the time. "The Germans were building rockets," he said, adding that the White House was a "bull's-eye."

President Harry Truman, Roosevelt's successor, expanded the bunker during his reconstruction of the White House, turning it into the bomb shelter it is today.

"The PEOC is designed to be a command center during emergencies, with televisions, phones, and communications facilities," former first lady Laura Bush wrote in her 2010 memoir.

She hunkered down in the bunker alongside senior White House staff after the 9/11 attacks in 2001 — perhaps the most well-known use of the bunker in its history. Then-President George W. Bush was in Florida at the time, Matt Novak reported for Gizmodo, but it's protocol for the Secret Service to take immediate family members down there. Photos from the US National Archives show other leaders at the time, such as Vice President Dick Cheney, Secretary of State Colin Powell, and national security adviser Condoleezza Rice, also sheltering in the bunker at the time.

"I was hustled inside and downstairs through a pair of big steel doors that closed behind me with a loud hiss, forming an airtight seal," Laura Bush wrote of the subterranean halls. "We walked along old tile floors with pipes hanging from the ceiling and all kinds of mechanical equipment."

She said she was taken to a small room with a conference table and shown a foldout bed that evening that George Bush, who had by then arrived from Florida, refused to stay overnight in.

It's not the White House's only bunker

The 9/11 attacks prompted national security officials to call for a more advanced bunker.

Ronald Kessler, the author of "The Trump White House: Changing the Rules of the Game," told Philip Bump of The Washington Post that national security personnel originally planned to evacuate White House staff and the presidential family to a remote location in the event of a nuclear attack. But 9/11, he said, made them realize escaping Washington while the country was under attack would be difficult. Roads would be too packed for vehicle travel, and a helicopter escape would be "very risky."

So plans for a bunker under the White House's North Lawn began to emerge in 2010, during the Obama administration. The General Services Administration went to great lengths to keep the project a secret, Bump wrote: They said the construction was to replace existing White House infrastructure, put up a fence, and ordered subcontractors to stay mum.

While these presidential bunkers are shrouded in secrecy, Popular Mechanics writer Caroline Delbert looked to other underground structures to get an idea of how they may have been built. Consider the European Organization for Nuclear Research's Large Hadron Collider, Delbert wrote: The process involved checking the area for archaeological artifacts and clearing the site for construction, flash freezing the water table (the next level below the ground) to build through, and building a 7-meter walled structure to protect nearby workers from radiation. All of it was cleared one small area at a time, she said.

Soldiers of the White House Military Office staff the bunkers every 12 or 24 hours, according to Delbert. Just how deep these bunkers go is also a mystery. "As the Union of Concerned Scientists points out, the highest yield nuclear warhead in the US arsenal can blast up to 1,000 feet deep," Delbert wrote. "The presidential bunker must be at least that far below the surface."

However, Kessler told Bump that the newest bunker, which is supposed to act as a command center and living quarters, is five stories underground with food and a self-contained air supply. It's also sealed off to prevent radiation from seeping in during a nuclear attack, he said.

But there are also underground tunnels, Kessler added, that allow the president, his family, and his staff to escape the White House entirely.