<P> In November 2010, workers began testing the North Pool waterfall, and construction progressed through early 2011 . In March installation of glass panels on the museum pavilion's façade began, and in May workers began testing the South Pool waterfall . Most of the memorial was finished in time for the 10th anniversary of the terrorist attacks, with the museum planned for completion one year later . By September 2, 243 trees were planted at the site and eight more were planted in the days before the memorial opened . By then, both pools were completed and the waterfalls were tested daily . </P> <P> On September 12, 2011, one day after the 10th anniversary of the September 11 terrorist attacks, the memorial opened to the public with a lengthy set of rules and regulations approved by the foundation's board of directors . The period from September 11, 2011 to May 25, 2014 was known as the "interim operating period", when the memorial was surrounded by construction of neighboring World Trade Center projects; the fence was taken down on May 25, 2014 . Three months after its opening, the memorial had been visited by over a million people . </P> <P> In January 2004, Reflecting Absence, by architect Michael Arad and landscape architect Peter Walker, was selected from 5,201 entries from 63 countries as the winner of the LMDC's design competition . Two 1 - acre (4,000 m) pools with the largest man - made waterfalls in the United States comprise the footprints of the Twin Towers, symbolizing the loss of life and the physical void left by the attacks . The waterfalls are intended to mute the sounds of the city, making the site a contemplative sanctuary . Landscape architect Peter Walker planted many parts of the memorial with white oaks . More than 400 swamp white oak trees fill the Memorial plaza, enhancing the site's reflective nature . </P> <P> Pedestrian simulations tested the memorial's design . The pedestrian - modeling program Legion was used to simulate visitor utilization of the space, and its design was tweaked to prevent bottlenecks . The fountain was engineered by Delta Fountains . </P>

What does the water at the 9 11 memorial represent