of . Before Burj Khalifa was completed, Willis Tower led in the height-to-tip category with after its antenna was extended in 2000, making Willis Tower slightly taller height-to-tip than the WTC north tower's antenna that measured . After the terrorist attacks on September 11, 2001, the WTC became the world's tallest two buildings to be destroyed or demolished. They took that distinction from the Singer Building, which stood tall until the late 1960s where One Liberty Plaza now stands right across Church Street from the WTC site. A different superlative for skyscrapers is their number of floors. The original World Trade Center set that record at 110 in the early 1970s, and this was not surpassed until the Burj Khalifa opened in 2010. Tall freestanding structures such as the CN Tower, the Ostankino Tower and the Oriental Pearl Tower are excluded from these categories because they are not "habitable buildings", which are defined as frame structures made with floors and walls throughout. History of record holders in each CTBUH category Tallest freestanding structures on land Freestanding structures must not be supported by guy wires, the sea or other types of support. It therefore does not include guyed masts, partially guyed towers and drilling platforms but does include towers, skyscrapers (pinnacle height) and chimneys. (See also history of tallest skyscrapers.) The world's tallest freestanding structure on land is defined as the tallest self-supporting artificial structure that stands above ground. This definition is different from that of world's tallest building or world's tallest structure based on the percentage of the structure that is occupied and whether or not it is self-supporting or supported by exterior cables. Likewise, this definition does not count structures that are built underground or on the seabed, such as the Petronius Platform in the Gulf of Mexico. Visit world's tallest structure by category for a list of various other definitions. The tallest freestanding structure on land is the Burj Khalifa in Dubai, United Arab Emirates. The building surpassed the height of the previous record holder, the CN Tower in Toronto, Ontario, on September 12, 2007. It was completed in 2010, with final height of . History The following is a list of structures that have held the title as the tallest freestanding structure on land. Notable mentions include the Pharos (lighthouse) of Alexandria, built in the third century BC and estimated between . It was the world's tallest