About 8:30 AM on September 11, 2001. Lower Manhattan, view from Brooklyn

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Slurry wall segment - The proximity of the Hudson River presented a significant challenge to the planners of the World Trade Center. Before excavation and construction could begin in 1966, it was necessary to find a way to prevent river water from seeping into or flooding the site. The innovative solution was to build a concrete retaining wall, known as a slurry wall. Despite fears that it might be breached on 9/11, thereby worsening the catastrophic impact of the attacks, the wall held. A portion of the original wall, which can be seen here, has been preserved.

The great slurry wall is the most dramatic element which survived the attack, an engineering wonder constructed on bedrock foundation... The foundations withstood the unimaginable trauma of the destruction and stand as eloquent as the Constitution itself asserting the durability of Democracy and the value of individual life. -Daniel Libeskind, World Trade Center Site Master Planner, 2002

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The Last Column

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Tieback - Recovered from the World Trade Center site after September 11, 2001.

Reinforcing tiebacks, each typically comprising 21 steel cables enclosed in a metal pipe, were drilled into the slurry wall and down into bedrock. This section of tieback was removed from the original slurry wall during construction of the 9/11 Memorial Museum.

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Dedication Pedestal - Recovered from the World Trade Center site after September 11, 2001.

Construction of the World Trade Center began on August 5, 1966, under the auspices of the Port of New York Authority (later known as the Port Authority of New York and New Jersey). The agency sought to create a contemporary international trade and business hub that would revitalize the economy of lower Manhattan and the metropolitan region.

The first tenants moved into 1 World Trade Center (North Tower) in December 1970 and into 2 World Trade Center (South Tower) 13 months later, before construction of either skyscraper had been completed. The world's tallest buildings were dedicated in a ribbon-cutting ceremony on April 4, 1973. This stainless steel pedestal was installed on the five-acre Plaza to commemorate the occasion.

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Section of steel facade, North Tower, floors 96-99 - Recovered from the World Trade Center site after September 11, 2001.

This piece of steel, once part of the north facade of the North Tower, was located at the point of impact where hijacked Flight 11 pierced the building at the 93rd through the 99th floors.

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Section of steel facade, North Tower, floors 93-96 - Recovered from the World Trade Center site after September 11, 2001.

On 9/11, hijacked Flight 11 tore into the north facade of the North Tower, creating a gash from the 93rd through the 99th floors and tearing apart steel columns weighing many tons. The underbelly of the aircraft mangled the top of this facade segment with force sufficient to twist and shred the steel.

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Trying to Remember the Color of the Sky on That September Morning - Spencer Finch, Watercolor on paper

This art installation, created in 2014, is composed of 2,983 individual watercolor drawings, each a distinct attempt by the artist to remember the color of the sky on the morning of September 11, 2001. Commemorating the people killed in the attacks of September 11, 2001 and February 26, 1993, every square is a unique shade of blue, combining to create a panoramic mosaic of color.

Finch's work centers on the idea of memory. What one person perceives as blue might not be the same as what another person sees. Yet, our memories, just like our perception of color, share a common reference.

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Reposed behind this wall are the remains of many who perished at the World Trade Center site on September 11, 2001

The Repository is maintained by the Office of Chief Medical Examiner of the City of New York

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No Day Shall Erase You from the Memory of Time

This quotation from Book IX of The Aeneid by the Roman poet Virgil suggests the transformative potential of remembrance. The same idea is expressed in each of the letters which have been forged from pieces recovered World Trade Center steel by New Mexico artist Tom Joyce.

Originally trained as a blacksmith, Joyce was invited to harness the transformative process that occurs when iron is touched by fire. He took wounded, remnant steel -- made of iron and carbon -- and forged it, by heating and folding, into letters of hope and beauty. The result reminds us that Virgil's words are not just a statement; they are a promise.

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Vesey Street stair remnant: "The Survivors' Stairs" -- Part 2

This staircase once connected the northern edge of the World Trade Center's Austin J. Tobin Plaza to the Vesey Street sidewalk below. On September 11, 2001, the stairs and an adjacent escalator provided an unobstructed exit for hundreds seeking to escape. To reach the stairs, many had to cross the Plaza beneath treacherous debris falling from the North Tower.

Go down this set of stairs and then just run, run as fast as you can. -David Brink, Lieutenant, New York City Police Department Emergency Service Unit, recalling what he said to evacuees on 9/11.

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Segment of radio and television antenna, North Tower - Recovered from the World Trade Center site after September 11, 2001.

A transmission tower approximately 360 feet tall, assembled atop the roof of the North Tower, began broadcasting television signals in 1980. Changes over time included the installation in 2000 of a high-definition TV master antenna. Transmissions for most station failed shortly after hijacked Flight 11 pierced the North Tower on 9/11. All transmissions ceased by 10:28 a.m., when the tower collapsed.

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Elevator motor - Recovered from the World Trade Center site after September 11, 2001.

The Twin Towers were the first skyscrapers to employ a system of local and express elevators, an innovation that reduced elevator travel time and made the buildings attractive to occupants of the upper floors. In addition to elevators servicing basement levels, each tower's 99 elevators included freight, local, and high-speed express cars. Two express cars traveled directly to the Windows on the World restaurant in the North Tower, and two went to the South Tower Observation Deck.

This elevator motor, the largest model in the world when installed, powered one of the express or service cars, which moved at a speed of 1,600 feet per minute. A total of 99 motors operated the elevator system in each tower.

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World Trade Center elevators on 9/11

The attack on the World Trade Center precipitated the worst passenger elevator disaster in history, claiming the lives of an estimated 200 of the more than 2,700 victims at the site. At 8:46 a.m., during rush hour for thousands using the elevators, hijacked Flight 11 struck the North Tower, severing elevator cables and trapping hundreds of people above the 93rd floor. Below the impact zone, most on floors lower than 92 were able to evacuate via the stairs.

In the South Tower, some occupants began an immediate evacuation, placing them inside elevators, or about to enter or exit the cars, when hijacked Flight 175 struck that building. Smoke rose through the elevator shafts, jet fuel poured down, and flames spread in all directions. At the fire command station in the lobby of each tower, personnel were unable to guide cars down to the lobby or to communicate with some individuals trapped inside because of the loss of electricity. Efforts to pry open newly installed safety locks succeeded in only a few cases.

The elevator started to fall. It burst into flames... I got hit in the face by a fireball that came in through the gap between the elevator doors... I knew that I was hurt. I did not have a clue how badly. -Harry Waizer, recalling being in an elevator on his way to his office on the 104th floor of the North Tower

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Ladder Company 3 truck, New York City Fire Department - Recovered from the World Trade Center site after September 11, 2001.

FDNY Ladder Company 3 is located in the East Village neighborhood of Manhattan. On September 11, 2001, led by its highly decorated captain, Patrick "Paddy" John Brown, the company asked a dispatcher to deploy its members to the World Trade Center disaster. After they parked the rear mount aerial ladder truck on West Street near Vesey Street, 11 members of the company, some of whom had just gone off duty after completing overnight shifts, entered the North Tower. They were among thousands of uniformed responders who collectively formed the single largest dispatch of nonmilitary emergency personnel in the history of New York City and the nation.

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Ladder Company 3

Assigned to aid in the evacuation of civilians in the North Tower on 9/11, members of FDNY Ladder Company 3 are known to have reached the 35th floor by 9:21 a.m. Captain Patrick "Paddy" John Brown, unable to communicate directly with the lobby command post, used a functioning office telephone to call a Manhattan dispatcher. He reported that burn victims and numerous others were making their way down the stairs and that he understood the fire to be above the 75th floor. In his last recorded transmission, Captain Brown said, "Three Truck, and we are still heading up."

All 11 responding members of Ladder Company 3 were inside the building and killed when it collapsed at 10:28 a.m.

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South Tower column - Recovered from the World Trade Center site after September 11, 2001.

This column once stood in the core of the South Tower, probably between floors 30 and 33. During the tower's collapse, extreme stresses caused this multi-ton piece of steel to fold over onto itself. Three of the four welds that held the column together split open.

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Fuel tank remnant

A tank, positioned on the buttress slab at the bottom of the original slurry wall, held diesel fuel for emergency generators. During cleanup of the World Trade Center site after 9/11, the tank was cut apart for removal. Its torched-out base was left in place.

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The Last Column: Marker of loss -- Part 2

The Last Column, a 58-ton, 36-foot-tall piece of welded plate steel, was part of Column 1001B, one of 47 columns that supported the inner core of the South Tower. When the South Tower collapsed, this remnant remained anchored in bedrock, buried beneath the wreckage.

At Ground Zero, Column 1001B helped to support a temporary haul road, known as the Tully Road, laid close to where the South Tower's elevators had been located. Many first responders who were killed in the tower's lobby were believed to be buried nearby. In March 2002, after the remains of some missing members of FDNY Squad 41 had been found in the area, a squad member painted "SQ 41" to mark the recovery. The marking is still visible at the center of the column.

Prayer cards and other memorial tributes were attached to the Last Column, along with signatures and inscriptions left by recovery workers.

Cut-down and removal ceremonies - On the evening of May 28, 2002, workers representing the trades at Ground Zero -- ironworkers, laborers, dock builders, and operating engineers -- were given the honor to make small cuts around the Last Column in order to free the column from its footing. In a private ceremony organized by construction workers, the Last Column was lowered onto a flatbed truck while bagpipers played "Amazing Grace." The column was then shrouded in black and draped with an American flag. Two days later, New York City and the nation publicly saluted the Last Column as it departed from Ground Zero. Police and Fire Department buglers played "Taps," bagpipers and drummers offered a rendition of "America, the Beautiful," and NYPD helicopters flew overhead as the column was driven out of the site, marking the end of the nine-month recovery period at Ground Zero.

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73 box columns -- so-called because of their rectangular shape and hollow center -- formed the one-acre perimeter of the World Trade Center's South Tower at bedrock. The column remnants along this corridor once rose to form the south face of the South Tower.

The tower soared more than a quarter mile into the sky, supported by these steel columns anchored 70 feet belowground. Recovery workers sheared the columns to their current height during cleanup operations following 9/11. Many 9/11 victims' family members joined with landmark preservationists to advocate for the column remnants' designation as permanent historic assets of the World Trade Center site.

This excavation reveals the actual depth of the foundational support of the box columns, which were set within a concrete base atop a bedrock layer of Manhattan schist.

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Excavation - To create the slurry wall, workers dug a trench around the perimeter of the site and filled it with a mixture of clay and water. This slurry stabilized the trench, allowing engineers to pump in concrete. Heavier than slurry, concrete sank to the bottom of the trench, displacing slurry from the ground up and hardening into a solid, watertight wall.

To anchor the towers to bedrock located more than 70 feet below street level, laborers excavated more than a million cubic yards of earth from the area inside the slurry wall. They deposited this fill into the adjacent Hudson River, expanding the island of Manhattan and providing the first 23.5 acres of landfill on which the World Financial Center and Battery Park City were later built.

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World Trade Center tridents - Recovered from the World Trade Center site after September 11, 2001.

Each of these steel columns is the base of a "trident," the three-branched architectural element that gave the Twin Towers their distinctive facades. These columns are two of 84 that formed the structural perimeter of the North Tower.

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