A new book says the White House has a massive secret bunker beneath its north lawn for doomsday scenarios, while staffers battle a more immediate menace — insects — with pressurized salt guns.

The bunker, built during former President Barack Obama’s administration, was toured by members of President Trump’s staff last year, author Ronald Kessler wrote in The Trump White House: Changing the Rules of the Game, which was released Monday.

Kessler, a former Washington Post reporter and author of several books on the Secret Service and national security, wrote that the facility is large enough to fit the White House workforce indefinitely.

A large north-lawn construction project began in 2010, officially to improve White House electrical wiring and air conditioning, though journalists long suspected the $376 million project involved a bunker.

The White House already had a bunker, under the East Wing, called the Presidential Emergency Operations Center, where Vice President Dick Cheney and other senior officials hid during the 9/11 terrorist attacks. The new facility is much larger, Kessler wrote.

“At least five stories deep, the bunker, which was completed near the end of Obama’s tenure, can house the staff of the entire West Wing indefinitely in the event of a weapons of mass destruction attack,” Kessler wrote. “After Trump became president, top staffers toured the bunker, whose existence is classified.”

A spokesperson for the Secret Service declined to comment on the bunker's existence, or Kessler's reporting that the Secret Service is actively surveiling approximately 100 people deemed to have uttered serious "Class III" threats against Trump.

Kessler wrote that Trump has received about as many threats each day — between six and eight — as did Presidents Obama and George W. Bush.

Most threats are deemed "Class II" — made by incarcerated or institutionalized people without the means to make good on their threats — or less-serious "Class I" threats, which may be uttered drunkenly or otherwise without intended action.

"For operational security purposes the Secret Service does not comment on specific White House security measures or protective intelligence matters," said Secret Service spokesman Mason Brayman.

Kessler interviewed Trump, members of his family, and many current and former White House officials for his book, which also reports that the president himself often is an anonymous quotes “senior White House official” in news reports.

Although prepared for a nuclear winter, Kessler wrote that White House staff struggle with a more pressing fight, against flies, with salt-powered guns.

Kessler wrote that a widely reported news story last year about Trump ordering then-chief of staff Reince Priebus to swat a fly was untrue.

“The story had its origin in the fact that the West Wing, built on a swamp, is beset by flies. Trump hates flies. Staffers use air-pressured salt guns called Bug-a-Salt to kill them. Priebus was attacking an especially annoying fly in the Oval Office when Trump said jokingly, ‘Kill it! Kill it!’”

Watch: Bug-a-Salt guns in action:

