Excavation began around the footprints of the twin towers on the foundations for two huge reflecting pools at the heart of the "Reflecting Absence" memorial. A contractor whose workers helped remove debris from the site in the months after the Sept. 11, 2001, attacks began work on more than 140 columns that will support the memorial, a museum and a visitors' center.
Officials hope the museum and the memorial, which sets reflecting pools with cascading waterfalls in spots where the twin towers stood, will open in 2009.
Construction began in March, then halted two months later after city and state leaders ordered a redesign of the memorial, saying its near-$1 billion cost had to be slashed.
A new design approved at the end of June cut the memorial's underground space and raised the names of the nearly 3,000 people killed to street level. The memorial costs are now closer to $650 million.
The trade center site's owner, the Port Authority of New York and New Jersey, took over construction and pledged more money to the project. The foundation in charge of private fundraising launched a national ad campaign this summer to raise an additional $170 million needed to build the memorial.