Fourteen tons of fireworks illuminated the New York night on May 24, 1883, to celebrate the completion of one of the greatest engineering feats of the Gilded Age—the Brooklyn Bridge. Billed as the “Eighth Wonder of the World,” the longest suspension bridge ever built at the time spanned the East River to link the twin cities of New York and Brooklyn.

But, as that day’s edition of the Brooklyn Eagle pronounced, “to every human undertaking there seems of necessity to be a dark side.” In the case of the Brooklyn Bridge it was the lives lost during its 14-year construction.

As first assistant engineer C.C. Martin told the Brooklyn Eagle, “Had we thought so many would have been injured we would have kept a list, but we never imagined any one would be hurt, or that the bridge would have occupied so long a time in building,”

Efforts to tally how many were killed vary. In his book The Great Bridge, author David McCullough writes that the construction took the lives of 21 men, most of them immigrants. In his account to the Brooklyn Eagle, Martin detailed the accidental deaths of 27 workers, although master mechanic E.F. Farrington estimated the number could be as high as 40.

Months before construction even began, the bridge project claimed its first victim—its visionary designer. On June 28, 1869, German-born civil engineer John A. Roebling was surveying the location of the bridge tower on a ferry slip along the Brooklyn waterfront when his right foot became caught on a rope and was crushed by a docking boat, resulting in the amputation of two toes. Less than a month after the freak accident, Roebling contracted tetanus and died, leaving his 32-year-old son Washington Roebling suddenly in charge of the mammoth project.

The first construction fatalities occurred on October 23, 1871, when a pair of derricks used to haul granite blocks to the top of the bridge tower on the Brooklyn side suddenly fell. A wooden boom sheared off the top half of rigger John French’s head, while a man named Dougherty was crushed by a derrick mast. John McGarrity died while attempting to leap to safety, and stonemason Thomas Douglas later succumbed to his injuries.

To construct foundations for the bridge towers, engineers sank a pair of watertight wood-and-steel chambers, called caissons, face down into the East River. Working with shovels and even dynamite to excavate the riverbed, so-called “sandhogs” toiled in stifling heat and at more than double the normal atmospheric pressure due to the compressed air pumped in to keep water out and allow workers to breathe.

The deeper the sandhogs burrowed, the more they began to experience strange muscular paralysis, slurred speech, vomiting, chills and excruciatingly sharp joint pains and stomach cramps upon ascending to the surface. Unbeknownst to the workers, the symptoms of this “caisson disease,” also known as “the bends,” were due to bubbling nitrogen in their bloodstream caused by rapid decreases in atmospheric pressure when resurfacing too quickly.

READ MORE: The Dark Story of How Scientists Used Goats to Solve the Bends

On April 22, 1872, German laborer John Myers became the first laborer to die from the bends after suffering abdominal pain and collapsing at home after his second day on the riverbed. Eight days later Irishman Patrick McKay died after resurfacing, and within a month Daniel Reardon, another Irishman, succumbed to the bends. After the deaths of these three sandhogs in quick succession, Washington Roebling suspended digging for the Manhattan tower and decided not to reach bedrock, fearing it could lead to 100 more fatalities.

Having spent untold hours below the surface monitoring the project, Roebling himself also suffered from the bends. As a result, he eventually became bedridden. Confined to his Brooklyn Heights bedroom with crippling pain, Roebling continued to supervise construction, watching with a telescope and conveying instructions through his wife, Emily.

Working at dizzying heights to construct the two bridge towers more than 275 feet above water, several laborers plummeted to their deaths while others were killed by falling stones and granite blocks. Irish-born mason Neil Mullen, a widower with five children, died three days before Christmas in 1877 when arches supporting the roadway on the Brooklyn anchorage gave way when temporary wooden supports were removed before mortar had properly set.

Months later, one of the strands from a bridge cable snapped from the Manhattan anchorage and struck Thomas Blake and Newfoundland native Harry Supple, causing them to fall to their deaths. One of the most gruesome casualties occurred in 1873 when German rigger Peter Cope had his leg caught in a rope that wound around a hoisting engine, crushing his limb and killing him almost instantly.

Death continued to stalk the span even after its completion. Seven days after its grand unveiling, people thronged the bridge for a Memorial Day stroll on its elevated promenade. During the late afternoon, an estimated crowd of between 15,000 and 20,000 on the bridge caused foot traffic to logjam at a narrow staircase on the Manhattan side when a woman slipped down the stairs and screams pierced the air. Suddenly, hordes pressed against the pedestrians on the staircases from both sides like a vice.

According to eyewitness accounts, people were packed so tightly that blood gushed from their noses and ears. “Within a few minutes there were piles of crushed and bleeding pieces of humanity at the foot of each flight of stairs and the panic-stricken crowd was trampling them to death,” reported the New York Tribune. Twelve people died as a result of the May 31, 1883, stampede on the Brooklyn Bridge.