On March 21, 1871, Henry Morton Stanley set out from the African port of Bagamoyo on what he hoped would be a career-making adventure. The 30-year-old journalist had arrived on the “Dark Continent” at the behest of the New York Herald newspaper, yet he wasn’t chasing any ordinary scoop. He had been placed in charge of a grand expedition to find the explorer David Livingstone, who had vanished in the heart of Africa several years earlier.

Stanley, a Welsh-born orphan who had previously fought on both sides of the American Civil War, took to the mission with gusto. Despite never having set foot in Africa before, he assembled a caravan of over 100 porters and struck out into the unknown. “Wherever [Livingstone] is, be sure I shall not give up the chase,” he later wrote to the New York Herald’s editor. “If alive you shall hear what he has to say. If dead I will find him and bring his bones to you.”

At the time that Stanley began his relief operation, Dr. David Livingstone was the most renowned of all the explorers of Africa. Among other exploits, the Scottish missionary and abolitionist had survived a lion attack, charted the Zambezi River and walked from one side of the continent to the other. In 1866, he embarked on what was supposed to be his last and greatest expedition: a quest to locate the fabled source of the Nile River. The mission was supposed to last two years, yet by 1871, nearly six years had passed with only a few scattered updates on Livingstone’s whereabouts. Many Europeans had given him up for dead.

Stanley knew that Livingstone had last been spotted in the vicinity of Lake Tanganyika, but reaching the area proved to be a monumental task. Between March and October of 1871, the New York Herald expedition endured repeated setbacks as it trudged through endless miles of swampland and jungle. Crocodiles and swarming tsetse flies killed their pack animals, and dozens of porters abandoned the caravan or died from illnesses.

Stanley himself was ravaged by dysentery, smallpox and a near-fatal case of cerebral malaria, yet he continued to urge his party forward at a breakneck pace. By the time they arrived at Ujiji, a remote village in what is now Tanzania, they had crossed more than 700 miles of territory.

On November 10, 1871, after hearing rumors of a white man living in Ujiji, Stanley donned his finest set of clothes and entered the town with a small band of followers. As crowds of locals gathered around them, Stanley spied a sickly looking European with an unruly beard and white hair. Sensing that he had found his man, he approached, extended his hand and asked a now-famous question: “Dr. Livingstone, I presume?” When the stranger answered in the affirmative, Stanley let out a sigh of relief. “I thank God, doctor, I have been permitted to see you,” he said.

As Stanley soon learned, Livingstone had been languishing in the heart of Africa for several years. His Nile expedition had been beset by thievery and mass desertions by his porters, and a succession of tropical diseases had sapped his strength and forced him to travel with Arab slave traders. He was wasting away in a small hut when the relief operation finally reached him.

Despite his failing health, Livingstone refused an offer to return home and resumed his search for the source of the Nile. After being resupplied by Stanley, he parted ways with his rescuers in March 1872 and made his way south to Lake Bangweulu in modern-day Zambia. His illnesses later caught up with him, however, and he died from malaria and dysentery on May 1, 1873.

Even as Livingstone’s career as an explorer was ending, Stanley’s was just beginning. The journalist became a celebrity after returning from the New York Herald expedition, and he later penned a bestselling book titled “How I Found Livingstone.” In 1874, having grown bored with his old reporter’s gig, he secured funding from the Herald and the London Daily Telegraph and returned to Africa to resume Livingstone’s unfinished explorations.

Stanley’s 1874 expedition would go down as one of the most audacious journeys in the history of African exploration. Over the course of 999 days, his party successfully trekked into the continent’s central watershed and scoured its lakes in a 24-foot boat.

Stanley became the first person to circumnavigate Lake Victoria, Africa’s largest body of water, and he later charted Lake Tanganyika before venturing 1,800 miles down the Congo River to the Atlantic Ocean. Roughly half of the expedition’s 227 members died from disease, drowning and repeated altercations with native tribes, but its geographical achievements helped make Stanley’s name as an adventurer. In 1878, author Mark Twain argued that “Stanley is almost the only man alive today whose name and work will be familiar one hundred years hence.”

While Stanley’s trans-Africa expedition cemented his reputation as the heir to Livingstone, his subsequent activities on the continent would forever tarnish his legacy. In 1878, the explorer signed on with King Leopold II of Belgium for a project to bring trade and Christianity to the African Congo.

The expedition was originally sold to Stanley as a sweeping humanitarian endeavor, yet in reality, King Leopold was only using charity as a screen to create a “Congo Free State” whose people and resources he would eventually exploit for his own enrichment. As the King’s agent, Stanley built roads, outposts and even a railroad, earning the nickname “Bula Matari,” or “Breaker of Rocks,” for his tireless construction efforts. Historians still debate how much he knew about Leopold’s true plans, but the infrastructure he created later helped facilitate years of forced labor and violence that may have led to the deaths of millions of Congolese.

The controversy only continued to follow Stanley in 1887, when he led an African expedition to rescue Emin Pasha, a German territorial governor who was under attack from Muslim rebels in southern Sudan. The journey proved to be a disaster on almost all fronts. Stanley split the expedition in half and eventually reached Pasha with the front column in 1888, but not before several hundred members of his party perished from disease and Pygmy attacks. Even more horrific were the atrocities committed by the expedition’s unsupervised rear column, whose members indiscriminately tortured and murdered countless Africans.

The Pasha expedition would be Stanley’s last. He returned to London in 1890, and later authored books and toured the lecture circuit before serving in the British parliament. He was widely hailed as a hero and even knighted by Queen Victoria, yet by the early 20th century, revelations about the brutality of the Congo Free State had permanently cast a shadow over his career. When the journalist-turned-explorer later died in 1904, his connection to the Congo atrocities saw him denied burial in Westminster Abbey alongside his old associate, David Livingstone.