Almost as soon as the World Trade Center’s Twin Towers fell on September 11, 2001, thousands of firefighters, police officers, construction workers, search-and-rescue dogs and volunteers headed to Ground Zero to look for survivors. Because they didn’t know how many people were trapped alive in the wreckage, firefighters and other rescue workers had to search carefully through the unstable piles of rubble for air pockets, called “voids,” where they might find people who had been unable to escape from the collapsing buildings. To be safe, they didn’t use any heavy equipment at first. Some dug with their bare hands, while others formed bucket brigades to move small amounts of debris as efficiently as possible.

First Responders and Volunteers at Ground Zero

An aerial view of Ground Zero after the September 11 attacks on the World Trade Center in New York City. U.S. Customs/Getty Images

Unfortunately, there were not many survivors to find: Two firemen were pulled from their truck in a cavity beneath some wreckage, and a few people were pinned at the edges of the pile. By September 12, workers had rescued all of the people who were trapped at the site. After that, the Ground Zero workers had a new and more heartbreaking mission: to sift carefully through the debris in search of human remains. The fallen buildings were unstable, and engineers worried that the weight of trucks and cranes would cause the wreckage to shift and collapse again, so the workers had to keep using the bucket brigades. Meanwhile, huge fires continued to burn at the center of the pile. Jagged, sharp pieces of iron and steel were everywhere. The work was so dangerous that many firefighters and police officers wrote their names and phone numbers on their forearms in case they fell into the hole or were crushed.

Did you know? Fires continued to burn in lower Manhattan for 99 days after the attack.

Eventually, the pile stabilized enough that construction crews could start using excavators and other heavy equipment. Ironworkers hung from tall cranes and cut the buildings down, one reporter said, “like trees.” Structural engineers worked to reinforce the giant concrete “bathtub” that formed the two-by-four-block foundation of the buildings and protected it from flooding by the Hudson River. Crews built roads across the site to make it easier to haul away the debris. (By May 2002, when the cleanup officially ended, workers had moved more than 108,000 truckloads–1.8 million tons–of rubble to a Staten Island landfill.) But the site was still dangerous. Underground fires continued to burn for months. Every time a crane moved a large chunk of debris, the sudden rush of oxygen intensified the flames. Downtown Manhattan reeked of smoke and burning rubber, plastic and steel.

READ MORE: How 9/11 Became the Deadliest Day in History for U.S. Firefighters

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Ground Zero: Environmental and Health Concerns

In fact, the site was awash in harmful fumes and toxic dust. Especially in the days immediately after the towers fell, when investigators estimated that only 20 percent of the workers at the site had masks that would protect their lungs, the air was filled with diesel exhaust, pulverized cement, glass fibers, asbestos, silica, benzene from the jet fuel and lead. On September 11 alone more than 300 workers sought treatment for eye and respiratory problems caused by the pollutants in the air. Soon the official workers at Ground Zero received masks and other protective gear, but volunteers and other workers–like the day laborers and undocumented workers who were hired to clean the dust from nearby office buildings–simply covered their faces with bandannas and hoped for the best.

In 2002, the Centers for Disease Control’s National Center for Environmental Health sponsored a comprehensive study of the health of the rescue and recovery workers at Ground Zero and 9/11-related illnesses. They found that many of the first responders had developed severe respiratory problems and had persistent symptoms of post-traumatic stress. Other studies agreed: Enormous numbers of workers had sore throats, trouble breathing and “WTC cough.” Science Daily reported that “New York City firemen and emergency personnel exposed to dust from the collapse of the World Trade Center buildings experienced a decrease in lung function capability equal to 12 years of age-related decline during the year following the 9/11 disaster.” Many were gravely ill with kidney problems, silicosis, lung cancer, leukemia and heart disease. Doctors and public health officials traced these illnesses to the polluted air at Ground Zero.

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In 2006, then-governor of New York George Pataki signed legislation aimed at expanding benefits for those whose deaths were linked to their cleanup work at the World Trade Center site. Initial efforts to pass a measure that would provide health monitoring and financial aid to Ground Zero workers on the federal level stalled. Finally, in January 2011, the James Zadroga 9/11 Health and Compensation Act, named after an NYPD officer whose death has been attributed to his work at Ground Zero, was signed into law.

The cleanup and recovery efforts at Ground Zero lasted for more than a year, with crews working around the clock. Construction workers found human remains in several places near the site of the Twin Towers in 2006, while the Environmental Protection Agency spent several years working to clean toxic dust out of downtown apartments. Still, dust and debris from the September 11th attacks will likely continue to affect downtown Manhattan for years; still, the impressive scale and speed of the cleanup work was a testament to the dedication of the workers and volunteers at the site.

Rebuilding efforts at the World Trade Center site continue. The centerpiece, a 1,776-foot tall skyscraper, opened in 2013, and the National September 11 Memorial and Museum opened in phases between 2011 and 2013.

9/11 Memorial

A competition was held to design a permanent memorial for the victims of 9/11. The winning design, Michael Arad’s “Reflecting Absence,” opened to the public on September 11, 2011, the 10 anniversary of the attacks. The eight-acre park features two reflecting pools with waterfalls in the footprint of the Twin Towers encircled by bronze panels featuring the names of all 2,983 people who died on 9/11. The National September 11 Memorial and Museum opened in May 2014.