Question: Liberty Park overlooks a memorial located at the World Trade Center site, which is the former location of what?
Context: The World Trade Center Site Memorial Competition was an open, international memorial contest, initiated by the Lower Manhattan Development Corporation (LMDC) according to the specifications of architect Daniel Libeskind, to design a World Trade Center Site Memorial (later renamed the National September 11 Memorial) on a portion of the World Trade Center site.  The competition began April 28, 2003 and the winner—Michael Arad and Peter Walker's "Reflecting Absence"—was revealed January 14, 2004 in a press conference at Federal Hall National Memorial in New York City.  The contest garnered 5,201 entries from 63 nations and 49 U.S. states, out of 13,683 registrants from all 50 U.S. states and 94 nations, making it the largest design competition in history. Liberty Street is a street in New York City that stretches east-west from the middle of Lower Manhattan almost to the East River.  It borders such sites as One Chase Manhattan Plaza, the Federal Reserve Bank of New York, One Liberty Plaza, Liberty Plaza Park, the World Trade Center site, the World Financial Center, Gateway Plaza, Liberty Park, and the North Cove marina.  A FDNY Firehouse, Engine Co. # 10 and Ladder Co. # 10, is located at 124 Liberty Street, directly across from Ground Zero. One World Trade Center (also known as 1 World Trade Center, 1 WTC or Freedom Tower ) is the main building of the rebuilt World Trade Center complex in Lower Manhattan, New York City.  It is the tallest building in the Western Hemisphere, and the sixth-tallest in the world.  The supertall structure has the same name as the North Tower of the original World Trade Center, which was destroyed in the terrorist attacks of September 11, 2001.  The new skyscraper stands on the northwest corner of the 16 acre World Trade Center site, on the site of the original 6 World Trade Center.  The building is bounded by West Street to the west, Vesey Street to the north, Fulton Street to the south, and Washington Street to the east. The National September 11 Memorial & Museum (also known as the 9/11 Memorial and 9/11 Memorial Museum) are a memorial and museum in New York City commemorating the September 11, 2001 attacks, which killed 2,977 victims, and the World Trade Center bombing of 1993, which killed six.  The memorial is located at the World Trade Center site, the former location of the Twin Towers that were destroyed during the September 11 attacks.  It is operated by a non-profit corporation whose mission is to raise funds for, program, own, and operate the memorial and museum at the World Trade Center site. 4 World Trade Center (also known by its street address, 150 Greenwich Street) is a skyscraper that is part of the World Trade Center complex in New York City.  It opened to tenants and the public on November 13, 2013.  It is located on the southeast corner of the 16 acre World Trade Center site, where the original nine-story 4 World Trade Center stood.  Pritzker Prize-winning architect Fumihiko Maki was awarded the contract to design the 978 ft building.  s of 2016 , it is the third tallest skyscraper at the rebuilt World Trade Center, behind One and 3 World Trade Center.  However, 2 World Trade Center is expected to surpass the height of both buildings upon completion.  The total floor space of the building includes 1.8 million square feet (167,000 square meters) of office and retail space.  The building's groundbreaking took place in January 2008. 7 World Trade Center (7 WTC) refers to two buildings that have existed at the same location within the World Trade Center site in Lower Manhattan, New York City.  The current structure is the second building to bear that name and address in the World Trade Center complex.  The original structure, part of the original World Trade Center, was completed in 1987 and was destroyed in the September 11 attacks.  The current building opened in May 2006.  Both buildings were developed by Larry Silverstein, who holds a ground lease for the site from the Port Authority of New York and New Jersey. The World Trade Center site, formerly known as "Ground Zero" after the September 11 attacks, is a 14.6-acre (5.9 ha) area in Lower Manhattan in New York City.  The previous World Trade Center complex stood on the site until it was destroyed in the September 11 attacks.  The Port Authority of New York and New Jersey (PANYNJ), Silverstein Properties, and the Lower Manhattan Development Corporation (LMDC) oversee the reconstruction of the site according to a master plan by Studio Daniel Libeskind.  The site is bounded by Vesey Street to the north, the West Side Highway to the west, Liberty Street to the south, and Church Street to the east.  The Port Authority owns the site's land (except for 7 World Trade Center).  Developer Larry Silverstein holds the lease to retail and office space in four of the site's buildings. 5 World Trade Center (also referred to as 130 Liberty Street) is a planned skyscraper at the World Trade Center in Lower Manhattan, New York City.  The site is across Liberty Street, to the south of the main 16 acre World Trade Center site.  The project is currently on standby while the Port Authority explores a potential sale of the lot to a developer and also finds tenants to occupy the skyscraper.  The proposed building shares its name with the original 5 World Trade Center, which was heavily damaged as a result of the collapse of the North Tower during the September 11 attacks and was later demolished.  The Port Authority has no plans to construct a building at 130 Liberty Street, although it is open to future development of the site as office, retail, hotel, residential or some mix of those uses. Liberty Park is a 1 acre elevated public park at the World Trade Center in New York City, overlooking the National September 11 Memorial & Museum.  It is located above the Vehicular Security Center and opened on June 29, 2016.  The St. Nicholas National Shrine is located within the park, as well as "The Sphere", the iconic sculpture salvaged from the World Trade Center site. The Twin Towers II (also known as Twin Towers 2, New Twin Towers, Trump Twin Towers and World Trade Center Phoenix ) was a proposed twin-towered supertall skyscraper complex which would have been located at the World Trade Center site in Manhattan, New York City.  The proposed complex would have replaced the former Twin Towers of the World Trade Center destroyed in the September 11 attacks, restoring the skyline of the city to its former state.  The main design for the proposed complex would feature new landmark twin towers, identical to the originals designed by Minoru Yamasaki, though it would feature 115 stories—5 floors taller than the originals, among other differences.  Beside the towers, an above-ground memorial would have occupied the footprints of the original towers.  The new site would also have featured three 12-story buildings, replacing the original 3, 4 and 5 World Trade Center.  The complex was designed and developed by American architect Herbert Belton and American engineer Kenneth Gardner, and sponsored by businessman and 45th President of the United States, Donald Trump.
Answer: Twin Towers that were destroyed during the September 11 attacks