In a quest to fulfill a centuries-old dream to connect the Atlantic and Pacific Oceans, the builders of the Panama Canal quickly learned that the construction of a waterway across a narrow ribbon of land looked easier on a map than in reality. The Panamanian isthmus proved to be one of the most difficult—and deadly—spots in the world in which to construct a channel. The builders of the passage attempted to re-engineer the natural landscape, but nature didn’t give up without a fight.

Construction crews literally had to move mountains in a snake-infested jungle with an average temperature of 80 degrees and 105 inches of rainfall a year. In the wet season, torrential downpours transformed the flood-prone Chagres River into raging rapids and soaked workers. “Sometimes you didn’t see sun for about two straight weeks,” recalled laborer Rufus Forde. “In the morning you had to put your clothes on damp. There was no sun to dry them.”

Death could strike in the form of an 18-ton boulder or miniscule, malaria-carrying mosquitoes that bred by the millions in festering swamps and puddles. Over the span of more than three decades, at least 25,000 workers died in the construction of the Panama Canal. “The working condition in those days were so horrible it would stagger your imagination,” recalled laborer Alfred Dottin. “Death was our constant companion. I shall never forget the train loads of dead men being carted away daily, as if they were just so much lumber.”

A French venture started construction of the Panama Canal in 1881. Seeking to duplicate his success in leading the construction of the Suez Canal, French diplomat Ferdinand de Lesseps found that building a 51-mile sea-level canal through Panama’s mountainous jungle would be far more difficult than a 120-mile passage through the flat Egyptian desert.

Ceaseless rains triggered mudslides that buried workers alive. Floods swept away construction equipment. On top of everything, an earthquake rocked the country, and fire destroyed the city of Colón when a civil war ignited. “There is too much water, the rocks are exceedingly hard, the soil is very hilly and the climate is deadly. The country is literally poisoned,” complained senior French engineer Adolphe Godin de Lépinay.

Outbreaks of dysentery and epidemics of yellow fever and malaria decimated the workforce. An estimated three-quarters of the French engineers who joined Lesseps in Panama died within three months of arriving. A Canadian doctor estimated that between 30 and 40 workers a day died during the wet seasons in 1882 and 1883, writes author Matthew Parker in Panama Fever.

By the time France abandoned the project in 1888, accidents and disease had claimed the lives of a staggering 20,000 laborers, according to the U.S. State Department. Most of the dead hailed from Caribbean islands such as Antigua, Barbados and Jamaica.

Sixteen years after the French venture went bankrupt, an ascendant United States restarted work on the partially excavated ditch. The Americans encountered many of the same obstacles as the French in the project’s first year as yellow fever and malaria killed hundreds of workers. As death pervaded the canal zone, downtrodden chief engineer John Findley Wallace made plans for his return trip home—by importing a metal coffin. By June 1905, three-quarters of the original American contingent had fled. Wallace followed suit and returned to the United States with his metal coffin—occupied by the corpse of one of his workers.

Wallace’s successor, John F. Stevens, emphasized the work undertaken by chief sanitary officer William Crawford Gorgas. For centuries, it was thought that filth, rotting garbage and airborne bacteria released from tropical soil caused yellow fever and malaria (derived from the Italian for “bad air”). A yellow fever survivor, Gorgas was among the doctors whose research pinpointed the role played by mosquitoes in spreading tropical diseases.

Spearheading a massive public health campaign in the canal zone, Gorgas ordered the fumigation of homes, the drainage of pools of water and the attachment of screens to windows and gutters. To smother mosquito larvae, health officials sprayed crude oil mixed with kerosene in water sources and puddles. Thanks to those efforts, yellow fever cases on the isthmus were largely eradicated by the end of 1905. Although case numbers dropped, malaria proved more stubborn. Sanitary inspector Joseph Le Prince estimated that 80 percent of the workforce was hospitalized at some point during 1906 for malaria. Still, Gorgas is credited with saving tens of thousands of lives.

With the yellow fever threat abating, accidents replaced disease as the largest cause of death in the canal zone in 1909. The most dangerous work took place as laborers carved a ditch 45 feet deep and at least 300 feet wide through an eight-mile mountainous stretch known as the Culebra Cut.

Nicknamed “Hell’s Gorge,” the Culebra Cut was a cauldron of noise with roaring locomotives and belching steam shovels where risks of death ranged from drowning to electrocution. Workers blasted away at the mountains with upwards of 60 million pounds of dynamite, which could ignite prematurely in the tropical Panamanian climate. Excavating machines also detonated unexploded charges as was the case in a December 1908 accident that killed 23 men.

Flooding regularly submerged equipment, and the unstable ground could give way at any instance. “The work of months or even years might be blotted out by an avalanche of earth,” lamented a senior U.S. administrator.

Particularly for workers partially deafened as a side effect of drinking quinine to ward off malaria, the inability to hear made deadly railroad accidents a regular occurrence. In an oral history, George Hodges remembered a fellow worker who fell trying to hop on a train and the wheel of another train “cut his body right in two…as if he had been chopped with a machete.”

Laborer Antonio Sanchez said work in the cut was like “going to a battlefield.” With workers suffering gruesome injuries, some of which required amputation, hospitals in the canal zone resembled those in a war zone. So many Panama Canal workers were maimed during the construction that artificial limb makers competed for highly coveted contracts with the canal builders. One such manufacturer, A.A. Marks, boasted that its waterproof arms and legs were “most suited to the climate and conditions of the locality” and the “only kind manufactured that would meet the demands” of injured workers who returned to the job on the Panama Canal.

Between 1904 and the end of construction in 1913, the United States recorded the deaths of 5,855 canal workers. When combined with the deaths from the French venture, Parker estimates it amounted to 500 lives lost for each mile of the canal.