In 1925, Colonel Percy Harrison Fawcett journeyed into the Brazilian jungle in search of ‘Z,’ an ancient lost city that he believed existed somewhere in the uncharted depths of the Amazon. The British-born explorer expected to make archaeological history, but following a final dispatch on May 29, he and his two companions vanished without a trace. Their disappearance inspired rampant speculation, and many would-be rescuers later died while trying to hunt down evidence of their fate. More than 90 years after the Fawcett expedition dropped off the map, take a look back at one of the great exploration mysteries of the 20th century.

With his steely blue eyes, manicured beard and trademark Stetson hat, Colonel Percy Fawcett looked like the quintessential swashbuckling adventurer. His resume included a stint as a British artilleryman in Sri Lanka, a tour of duty in World War I and a top-secret gig as a spy in Morocco. He was most famous for his half-dozen mapmaking expeditions to the wilds of the Amazon, a place he called “the last great blank space in the world.” Beginning in 1906, Fawcett had ventured into previously uncharted territory in Brazil and Bolivia, where he dodged poisonous pit vipers and made contact with hostile native tribes. His exploits grabbed headlines around the world and won him a prestigious medal from the Royal Geographical Society. They even inspired Sir Arthur Conan Doyle to write the 1912 novel “The Lost World.”

Along with making his name as one of the world’s great explorers, Fawcett’s adventures also led him to develop the theory that an advanced and ancient city lay in the Amazon. His run-ins with native Indians had convinced him that it was possible for large groups to thrive in the unforgiving environment of the rainforest, and he’d stumbled upon references to sophisticated settlements in the histories of the European Conquistadors. He was particularly enthralled by a Portuguese fortune hunter’s 1753 account of a stone jungle metropolis of great “size and grandeur.”

As the years passed, Fawcett became increasingly obsessed with seeking out his modern day El Dorado, which he dubbed the city of  “Z.” He launched two searches for it in the early 1920s, but was driven out of the jungle on both occasions by poor weather, fever and exhaustion. It took more than three years of campaigning before he finally secured funding for a third mission. Despite warnings that he was taking off on a fool’s errand, the 57-year-old explorer remained convinced that Z was lurking somewhere in the unexplored Mato Grosso region of Brazil.

Fawcett had no shortage of volunteers for his final expedition, but he turned down the likes of T.E. Lawrence—the famed adventurer known as “Lawrence of Arabia”—in favor of taking along his 21-year-old son, Jack, who shared his near-religious zeal for the Z theory. Rounding out the party was Jack’s best friend, Raleigh Rimell. After loading up on mosquito netting, canned food, machetes and other provisions, the trio set sail from New Jersey in January 1925. “We shall return,” Fawcett vowed to reporters, “and we shall bring back what we seek.”

The Fawcett expedition first sailed for Rio de Janeiro before trekking inland to the remote Amazonian outpost of Cuiabá, where they purchased pack animals and hired a pair of native guides. On April 20, 1925, they ventured into the jungle for the first time. Ahead of them lay a sweltering maze of dense undergrowth, piranha-infested rivers and unmapped territory populated by hostile native tribes. During the expedition’s early weeks, however, it was the insects that proved the most pressing threat. Swarming mosquitoes and blood-sucking gnats made sleep difficult and travel miserable, and Rimell’s foot became severely swollen from tick bites. Undeterred, Fawcett set a demanding pace of between 10 and 15 miles a day. During one leg, he got so far ahead of his young companions that he was forced to camp alone for a night.

On May 29, the team reached “Dead Horse Camp,” the spot where Fawcett had been forced to shoot his spent horse and call it quits during one of his earlier searches for Z. There, they unloaded their equipment and sent their guides back to Cuiabá. Before the natives left, Fawcett handed over the last of the expedition’s dispatches. Among them was a letter to his wife, Nina. “Jack is well and fit and getting stronger every day,” it read. “You need have no fear of any failure.” At that, the trio struck off into the bush alone.

Fawcett had warned that his expedition would go dark once it entered uncharted territory, but by 1927, nearly two years had passed with no word from the Colonel or his young companions. Newspapers that had previously hailed Fawcett as being impervious to the perils of the jungle began speculating that he was dead, and witnesses surfaced with bewildering rumors about his whereabouts. One man claimed Fawcett had gone native and was living in the jungle; another, that he was being held prisoner by Indians. Still another maintained that he had become chief of a tribe of cannibals along the Xingu River.

In 1928, the Royal Geographical Society’s George Miller Dyott launched the first expedition to search for Fawcett and his party. He emerged from the jungle convinced that expedition had perished, but he had no hard evidence and was unable to locate any bodies. “There is consequently still no proof that the three explorers are dead,” a defiant Nina Fawcett told reporters. She remained hopeful of her son and husband’s return until her death.

Amazon jungle in Brazil.
Dado Galdieri/Bloomberg via Getty Images

In the years since the Dyott expedition, the mystery surrounding Fawcett’s disappearance has lured scores of other would-be rescuers and investigators into the Amazon. It’s estimated that as many as 100 of them have died in the jungle, and a few have followed in the explorers’ footsteps by vanishing without a trace. As recently as 1996, a team of Fawcett-hunters led by a wealthy businessman named James Lynch was captured by Amazonian Indians and held for ransom. They only escaped with their lives after giving up $30,000 worth of equipment.

What really happened to the Fawcett expedition? Researchers have blamed its disappearance on everything from malaria and parasitic infection to starvation, drowning and jaguar attacks. Some have even argued that Fawcett—a longtime dabbler in mysticism—vanished on purpose and set up an occult commune in the jungle. A rare clue surfaced in 2005, when the journalist David Grann retraced Fawcett’s path through the Amazon. During a meeting with Kalapalo Indians, he learned that the tribe had preserved the tale of a meeting with the explorer in their oral history. The Indians claimed Fawcett had disregarded their warnings and trekked into the domain of a warlike tribe the Kalapalos called the “fierce Indians.” When the white men failed to return, the Kalapalos concluded that they had been ambushed and killed.

Fawcett’s fate may never be known for sure, but in recent years, evidence has shown that his theory about a sophisticated jungle city was not a total fantasy. As Grann points out in his book “The Lost City of Z,” many archeologists now believe the Amazon was home to dozens of bustling settlements in the centuries before the arrival of Europeans. Excavations have revealed the ruins of garden cities with earthen defensive walls, complex road networks and enough space for thousands of inhabitants. Some of these sites are nestled deep in the modern day state of Mato Grosso—the very region where Percy Fawcett hoped to find his mythical city of Z.