By Ian Bush

PHILADELPHIA (CBS) — From “duck and cover’ to “fire and fury”: as North Korea and the U.S. exchange nuclear threats, some of our government’s Cold War contingency plans still apply.

Most “fallout shelter” signs you see these days are rusted relics of the atomic age — not unlike the bunkers themselves, many of which have been repurposed for more modern needs.

But throughout that anxious era, a team of park rangers stood ready to evacuate the Liberty Bell were Philadelphia to come under surprise Soviet attack. A truck was on standby, and the drill was well-practiced.

“There were rankings of historical artifacts,” says Garrett Graff, author of “Raven Rock: The Government’s Secret Plan to Save Itself While the Rest of Us Die.” “There were numbered lists so that people knew at the Library of Congress, for instance, that they were to save Lincoln’s Gettysburg Address before they saved George Washington’s military commission. And that the Declaration of Independence would be saved before the Constitution.”

READ: South Jersey Used To Be Last Line Of Defense Against Nukes

No surprise: the National Park Service won’t reveal what security procedures are in place today at Independence National Historical Park. A spokesman says the original idea was to have a bombproof underground structure into which the Liberty Bell could be lowered. It was never built.

The Cold War-era plans for the icon of freedom, though, appear to be defunct, as is the nuclear bunker on Peanut Island in Palm Beach Harbor: it was constructed for President Kennedy’s use when he was at his “winter White House” and happens to be a stone’s throw from President Trump’s Mar-a-Lago resort.

But not all the old precautionary measures have been lost to history.

READ: North Korea Military: Trump’s Threat A ‘Load Of Nonsense’

One is a three-hour drive from Philadelphia, along the Pennsylvania border with Maryland. Another, in Northern Virginia.

“They’re really small cities built inside hollowed-out mountains,” says Graff.

Underground bunkers, staffed round-the-clock in case of catastrophic attack — but they’re not for most of us.

“Raven Rock in Pennsylvania would be the fallback location for the Pentagon, would be where the military would evacuate,” Graff explains. “Mount Weather, in Virginia, would be where the civilian government — the cabinet agencies, the president, the Supreme Court, the congressional leadership — would be evacuated in the event of an emergency or catastrophic attack on the Capitol.”

READ: Expert: 3 Events Need To Take Place Before War Breaks Out With North Korea

Each is built to house and sustain around 4,000 people for weeks at a time.

“They date back to early years of the Cold War but are still kept ready and operating with a skeleton crew — 24 hours a day, 365 days a year,” Graff says.

An Air Force runway in Nebraska is home to the president’s doomsday jet, the airborne nuclear command post. Its engines are always running.

“There’s a whole massive infrastructure that moves with the president wherever he goes, including emergency evacuation helicopters and other backup planes for Air Force One in the event that something happens to the main presidential aircraft,” Graff notes. “But what a lot of people forget about is effectively, these ‘shiny presidential toys’ are really tools to ensure the President of the United States is in communication with the nation’s nuclear arsenal — that he can launch a nuclear war — from wherever he is.”