Everything You Need to Know About the State of the Union Designated Survivor

— -- During President Obama’s State of the Union address on Tuesday night, most of our nation’s leaders in government -- members of Congress, Supreme Court Justices and almost all of the president’s Cabinet -- will cram into the chamber of the U.S. House of Representatives to hear the commander in chief outline his 2015 agenda.

Security, of course, will be tight. But if the unthinkable happens -- the Capitol is attacked, wiping out everyone inside the chamber -- there’s one Cabinet secretary surrounded by Secret Service agents somewhere else, waiting in the wings to become president.

He or she is known as the designated survivor, and it’s one of the most fascinating and mysterious jobs in Washington:

WHO IS THE DESIGNATED SURVIVOR?

Selected by the White House chief of staff, the designated survivor is a Cabinet-level official who spends the evening away from the Capitol, ready to take the reins if everyone above him or her in the presidential line of succession dies in a crisis at the House chamber.

For security purposes, the identity of the survivor is generally kept secret until the day of the address.

WAIT, WHAT’S THE PRESIDENTIAL LINE OF SUCCESSION ANYWAY?

As anyone who passed 10th-grade history knows, if the president dies or is removed from office, he or she is succeeded by the vice president, followed by the speaker of the house (currently Republican Rep. John Boehner) and then the president pro tempore of the Senate (Republican Sen. Orrin Hatch).

But thanks to the Presidential Succession Act of 1947, there’s a much more extensive contingency plan. If the president, vice president, speaker and Senate president pro tempore all die, the line of succession is as follows: the secretaries of state, treasury and defense; the attorney general; and the secretaries of the interior, agriculture, commerce, labor, health and human services, housing and urban development, transportation, energy, education, veterans affairs and homeland security.

WHEN DID WE START THIS GRIM TRADITION?

The practice likely began in the 1960s, according to historians. The nation, rocked by the Cold War, was for the first time facing the fear of a nuclear attack. But designees didn’t become public record until the 1980s.

HOW DO YOU PREPARE TO BECOME COMMANDER-IN-CHIEF?

Because of security concerns, the designated survivor procedures are kept under tight wraps, but we do know the survivor begins training immediately after he or she is selected a few weeks before the State of the Union speech.Past designated survivors say they were shown the Situation Room and briefed on continuity of government.

WHAT HAPPENS TO THE DESIGNATED SURVIVOR ON THE NIGHT OF THE SPEECH?

Before 9/11, the night was, by all accounts, relatively relaxed. One survivor said he spent the night with his daughter, another hosted a pizza party in the White House.

But post-9/11, things got a lot more serious. These days, the designated survivor is given presidential-level security for the evening, escorted to an undisclosed location and accompanied by a military aide carrying “the football,” a briefcase with the nuclear launch codes.

For a few hours, he or she is one the best-guarded individuals in the world.

WHO ARE OBAMA'S PAST DESIGNATED SURVIVORS?

Past designated survivors include Energy Secretary Ernest Moniz, Energy Secretary Steven Chu, Agriculture Secretary Tom Vilsack, Interior Secretary Ken Salazar, Housing and Urban Development Secretary Shaun Donovan and Attorney General Eric Holder.

WHO WON’T BE THE DESIGNATED SURVIVOR?

The designated survivor tends to be lower ranking in the line of succession -- the secretaries of state and the treasury and have never been chosen.

As secretary of the interior, Sally Jewell would normally be eighth in the line of presidential succession. But because she was born in the United Kingdom, she isn't eligible to be president and therefore will never be tapped as the designated survivor.

You’ll likely see Jewell in one of the front rows on Tuesday night.