The Great Pyramids of Giza are some of the largest, most foreboding structures on the planet, but the mystery of how they were built still plagues historians today. Some Egyptologists think aliens provided advanced technology, while engineers see evidence they were ancient nuclear power plants.

Today, the health risks associated with smoking cigarettes aren’t a secret. But since the mid-1900s, researchers have pointed out the harmful effects of these cancer sticks.

In the 20th century, the mass production of cigarettes fueled a global addiction — but Big Tobacco had a problem. Cancer rates were skyrocketing, and smoking-related illnesses became the leading cause of preventable death in America. This is the story of how an industry in peril conspired to survive — by keeping generations of smokers in their grip.

The deaths of the Dyatlov Nine didn’t just seem bizarre — they seemed violent. What could explain the brutal injuries, missing tongue, radioactive clothing, and burnt trees?

In winter 1959, a group of experienced mountaineers went missing in a remote part of Siberia. When their bodies were eventually found, their injuries — and radioactive clothing — confounded investigators. The incident became one of Russia’s most enduring mysteries.

When President Woodrow Wilson suffered a paralyzing stroke in 1919, First Lady Edith Wilson stepped in to maintain order in the White House.

Around the 9th century C.E., a new pope rose to power. For years, the congregation was certain Pope John filled the role perfectly... until he went into labor during an Easter processional. Had a woman ascended to the papacy in secret?

When critics didn’t understand Stanley Kubrick’s 1980 horror masterpiece, fans stepped in to explain the secrets behind its signature dread.

The release of The Exorcist in December 1973 caused a global wave of panic and trauma. Due to a series of on-set accidents and untimely deaths, many believe the film was cursed.

Nearly two decades after the Waco Siege, people are still wondering what transpired at the Branch Davidian compound.

On February 28th, 1993, government agents raided a compound in Waco, Texas, belonging to a religious group known as the Branch Davidians. A 51-day standoff ensued, ending in a violent siege.

Valiant Thor's story has sparked a flurry of theories regarding our government's knowledge of UFOs.

In 1957, the Pentagon received an unexpected visitor. His name was Valiant Thor, and allegedly, he was from Venus. His mission on Earth was to convince world leaders to deactivate their nuclear arsenals.

For centuries, people have debated whether or not Jesus Christ rose from the dead. From graverobbers, to Jewish High Priests, to Jesus's own disciples — a flurry of potential thieves has been placed at the scene of his empty tomb.

In Turin, Italy there’s a piece of cloth that has perplexed religious experts for decades. Is the Shroud of Turin one of the greatest antiquities known to us? Or nothing more than a medieval hoax?

Locals are sure that big corporations are after their coal, and started the Centralia Fire to smoke them out. Others insist the blaze was nothing more than bureaucratic incompetence.

For nearly 60 years, a fire has been raging beneath a small town in Pennsylvania. Centralia was once an epicenter of coal mining. But after the blaze sparked in 1962, toxic fumes and sinkholes forced residents to evacuate.

In the final episode of our 9/11 series, we examine the legacy of that day, and how its memory impacted — and still impacts — the world.

How much did the U.S. government actually know about the 9/11 attacks before they happened?

Throughout the 1990s, a private equity firm called the Carlyle Group began purchasing and investing in a multitude of private defense contracting and weapons companies.

We trace the path through history to 9/11, then examine how the official 9/11 Commission Report gave conspiracy theorists a foothold.

The horrific attacks of September 11th come to an end. In the aftermath, rescuers and volunteers clean up while military and government leaders decide how to respond.

The morning of September 11, 2001 changed the world. Twenty years later, questions still linger about the worst terrorist attack in U.S. history.

When U.S. forces invaded Iraq in 2003, investors began buying up Iraqi currency. Some expected a massive payup, others were scammed by unethical dealers. And some believe that terrorist cells or an international alliance of government agencies may be manipulating the dinar's value to reshape the global economy.

In the case of the century, O.J. Simpson was found not guilty of double homicide in 1995, despite a wealth of DNA evidence.