Long before the World Trade Center became synonymous with the
most damaging terrorist attack in U.S. history, it was a symbol
of engineering brilliance. Upon its completion in 1973, the two
towers that rose from the 16-acre (64749.7-square-meter) complex
consisting of seven different buildings in lower Manhattan were
the tallest structures in the world. But the construction of
such mammoth structures had its challenges. The first major
challenge was the building site itself. The location selected
for the project, on Manhattan's Lower West Side, had been built
upon generations of landfill that had actually grown and
compacted on itself so much that it had extended the Lower West
Side of Manhattan into the Hudson River. To reach a solid base
of bedrock, workers had to dig down 70 feet (21.3 meters). But
because of the proximity of the river, a barrier needed to be
created that would keep the excavated section of the city from
filling with water as fast as the earth was removed. The answer
was something that bec