With the perimeter walls secured in place, the crew could begin excavating the foundation site. They ended up digging up more than 1 million cubic yards of fill, which they dumped in the Hudson, extending the shore. The excavation actually added 28 acres of prime New York real estate, forming what is now Battery Park City.
When they had dug down to the bedrock, they blasted away large pits for the towers' support structure and set about building the massive foundation structure for the buildings above. Additionally, the basement structure had seven levels of usable space, which housed parking decks, stores and subway stations.
Putting the Twin Towers up was a major logistical challenge, in addition to a mind-boggling engineering problem. The buildings required a massive amount of steel -- some 200,000 tons total -- but the construction site only had room for a little bit at any one time. In order to keep construction moving without taking up too much construction site space, the Port Authority had to institute "just in time steel delivery."
In this system, all the steel was transported from the manufacturers to a giant railroad yard in New Jersey. Every major piece of steel was marked with a long ID number, indicating where and when it would be used. According to the construction schedule, the Port Authority would ship the steel pieces from the yard to the site exactly when it was needed -- smaller pieces went by truck and larger pieces by tugboat.
The construction process worked from the inside out. First, the crew built the steel framework of the inner core to a particular height, and then assembled the perimeter wall around it. The perimeter structure was actually formed from pre-fabricated sections of vertical columns attached to horizontal beams (called spandrels). The prefabricated sections were about 10 feet (3 m) wide, either two or three stories high, and weighed about 22 tons.
The floor structure was then installed between the outer perimeter wall and the inner core. The floors also came in pre-assembled sections, consisting of 32-inch-deep (81-cm) trusses topped with a corrugated metal surface. To finish each floor, the crew would pour concrete over the metal surface and top it off with tile. The floor sections included pre-assembled ducts for phone lines and electrical cable, to make things easier for the electricians who would come in later. After the steel structure was in place, the crew attached the outer "skin" to the perimeter -- anodized aluminum, pre-cut into large panels.
This continued, section by section, as the towers climbed higher and higher. The crew lifted the steel sections into place using four large cranes (four per tower), mounted to long steel structures fitted inside the tube structure. The cranes could actually lift themselves higher, using heavy hydraulics, as the floors were finished.
While the crew kept building upward, other workers started to flesh out the floors below, down to installing blinds and painting the walls. A number of businesses actually moved into their new WTC offices years before the towers officially opened.