Brad Meltzer can pinpoint the moment he became obsessed with conspiracy theories. His 11th-grade history teacher had just announced every high schooler’s dream: movie day.

“And then she put on this Kennedy assassination film,” the best-selling author and host of the HISTORY series Decoded and Lost History says. “And not one of those kooky, crazy ones, but one that actually asked the questions that should be asked… I remember it was one of the first times I felt like the foundations of my brain were kind of kicked aside and kicked open because it made me realize that not everything in the world is in the history books they give us in school.” recently caught up with Meltzer, whose latest books include The Lincoln Conspiracy: The Secret Plot to Kill America's 16th President and The First Conspiracy: The Secret Plot to Kill George Washington.

Specifically, we connected with him about his book The 10 Greatest Conspiracies of All Time: Decoding History's Unsolved Mysteries, for a little insight on four lingering enigmas we just can’t shake. From rumors that there actually isn’t any gold in Fort Knox to mysteries surrounding America’s version of Stonehenge to the disappearance of the White House cornerstone to Leonardo Da Vinci’s prophetic visions, he fills us in on his research.

The book explores whether there’s actually any gold at Fort Knox, the U.S. Bullion Depository. In fact, at one point you had it all set up to film inside when the secretary of the treasury canceled the shoot at the last minute. What do you think is going on there?

What was really amazing to us was how many people who had worked in Fort Knox or said they had a relative in there had said: There’s nothing there. You add those two things together—personal eyewitness accounts mixed with the government truly unwilling to let us show what is ours—and we want to know, why can’t we just prove what is there?

One of my favorite things in the book, though, was when we got to uncover what was held at Fort Knox at different times in history. It’s not only been a home for our gold, but also other items the government has stored secretly. In the 1940s, it held the Magna Carta, which we held for the British to keep out of the hands of the Nazis. The vault there also held the Declaration of Independence, the U.S. Constitution, the Gettysburg Address and three copies of the Gutenberg Bible. In the 1950s, the vault became part of the Cold War era and held drugs, vaccines and morphine in case of mass casualties.

The Georgia Guidestones in Atlanta, which some call America's Stonehenge, are a relatively modern mystery. What’s known about them and their intent?

Built in 1980 by a man who still remains anonymous and doesn’t say his true purpose, it has a half-million dollar price tag and all we know is they’re composed of 119 tons of solid granite and have coded messages that are engraved into them in the world’s eight most commonly spoken languages. The way the pillars are erected track astrological and solar cycles. Some say they’re the end of days, others say they call for genocide. But, my God, if you’re spending half a million in Atlanta, it begs the question, what are they about?

The east and west corners of the monument track sunrise and sunset. A slot that’s cut into one of the slabs marks the winter and summer solstices. A shaft that’s drilled through the center of the stone marks Polaris, the north star. And a slit that’s cut through the capstone marks the perfect noon time. So that means it’s this sort of granite Swiss Army knife: The Guidestones have a calendar, a compass, a translator and a guide...which is bananas.

What I think is most interesting now is what’s actually written on those stones. They say we should maintain humanity under 500 million, we should reproduce wisely, we should unite humanity with a new language, we should rule passion, faith and tradition all with tempered reason, and protect people in nations with fair laws and just rewards. In the middle of the time of Covid, in 2020, the Georgia Guidestones take on a whole new meaning.

The book also has a chapter on the disappearance of the White House Cornerstone. Why do you find it so compelling?

In 1792, stonemasons lay a cornerstone for the White House, the very first piece of the most important building at that point in Washington, D.C. And to this day, descriptions of the stone vary. Some people say it was small enough to carry, some say it was massive. The men who placed it were part of a secret organization, the Freemasons, and 24 hours after they put it down in an elaborate Freemason ceremony, the stone vanishes.

Some say it’s missing, some say it’s stolen, but the rumors about the stone itself get even wilder. In some descriptions the stone is inscribed by our Founding Fathers; others say it’s hollow and has secret documents of unimaginable wisdom. Some people say that it was stolen by the Freemasons themselves. (Of course the masons deny any involvement.) But for more than 200 years now the location of the cornerstone has been a mystery.

[The book] dives into the Freemasons, which is to me one of the most amazing stories in American history… The Freemasons include eight signers of the Declaration of Independence, nine signers of the U.S. Constitution and 15 U.S. presidents. Benjamin Franklin, Mark Twain, John Wayne, Bob Dole, Jesse Jackson, even Harry Houdini—all Freemasons. But what is also fun is that there’s another cornerstone that’s also missing—and that’s the cornerstone of the U.S. Capitol.

Your chapter on Leonardo Da Vinci digs into theories that the artist was a prophet. What did you discover?

It begins with the Codex Atlanticus—a codex is a collection of ancient works. So Da Vinci keeps all of these notebooks, fills thousands of loose pages with all the ideas he comes up with—every observation he’s ever made. But page 1,033 of the Codex Atlanticus goes missing. And everyone wants to know what’s on this page? Why is this page so important? Many thought it was lost history, but (it was found) and it turned out it was nothing less than the earliest known self-portrait of Da Vinci himself.

That image and the message that’s on it took us on this journey none of us expected. The beautiful part of trying to have your own Da Vinci code is seeing what else Da Vinci unleashed on the universe. His inventions are staggering. If you look at his drawings, he created a helicopter, a tank, scuba gear, a submarine, machine guns, he makes a parachute even though there’s no such thing as an airplane. And for me what’s inspiring about him is that every single one of the things he creates? None of them work. The scuba gear is made out of leather, it sinks. The submarine, you’d drown in it. The parachute doesn’t really work. But in time, all of it works and that’s what makes him so amazing. It wound up taking us down this path of different true-life Da Vinci codes.

Interview was edited for space and clarity.