<P> In March 1965, the Port Authority began acquiring property at the World Trade Center site . Demolition work began on March 21, 1966, to clear thirteen square blocks of low rise buildings in Radio Row for construction of the World Trade Center . Groundbreaking for the construction of the World Trade Center took place on August 5, 1966 . </P> <P> The site of the World Trade Center was located on landfill with the bedrock located 65 feet (20 m) below . To construct the World Trade Center, it was necessary to build a "bathtub" with a slurry wall around the West Street side of the site, to keep water from the Hudson River out . The slurry method selected by Port Authority's chief engineer, John M. Kyle, Jr., involved digging a trench, and as excavation proceeded, filling the space with a "slurry" mixture composed of bentonite and water, which plugged holes and kept groundwater out . When the trench was dug out, a steel cage was inserted and concrete was poured in, forcing the "slurry" out . It took fourteen months for the slurry wall to be completed . It was necessary before excavation of material from the interior of the site could begin . The 1,200,000 cubic yards (920,000 m) of material excavated were used (along with other fill and dredge material) to expand the Manhattan shoreline across West Street to form Battery Park City . </P> <P> In January 1967, the Port Authority awarded $74 million in contracts to various steel suppliers . Construction work began on the North Tower in August 1968, and construction on the South Tower was under way by January 1969 . The original Hudson Tubes, which carried PATH trains into Hudson Terminal, remained in service during the construction process until 1971, when a new station opened . The topping out ceremony of 1 WTC (North Tower) took place on December 23, 1970, while 2 WTC's ceremony (South Tower) occurred later on July 19, 1971 . The first tenants moved into the North Tower on December 15, 1970, while the South Tower started accepting tenants in January 1972 . When the World Trade Center twin towers were completed, the total costs to the Port Authority had reached $900 million . The ribbon cutting ceremony was on April 4, 1973 . </P> <P> In addition to the twin towers, the plan for the World Trade Center complex included four other low - rise buildings, which were built in the early 1970s . The 47 - story 7 World Trade Center building was added in the 1980s, to the north of the main complex . Altogether, the main World Trade Center complex occupied a 16 - acre (65,000 m) superblock . </P>

When were the twin towers built before 911
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