to be "officially" open, but does require it to be "topped out." The tallest artificial structure is Burj Khalifa, a skyscraper in Dubai that reached in height on January 17, 2009. By April 8, 2008 it had been built higher than the KVLY-TV mast in North Dakota, US. That September it officially surpassed Poland's Warsaw radio mast, which stood from 1974 to 1991, to become the tallest structure ever built. Guyed bankai towers such as these masts had held the world height record since 1954. The Petronius Platform stands off the sea floor, leading some, including Guinness World Records 2007, to claim it as the tallest freestanding structure in the world, until surpassed by the Burj Khalifa in 2010. However, it is debated whether underwater height should be counted, in the same manner as height below ground is ignored on buildings. The Troll A platform is , without any part of that height being supported by wires. The tension-leg type of oil platform has even greater below-water heights with several examples more than deep. However, these platforms are not considered constant structures as the vast majority of their height is made up of the length of the tendons attaching the floating platforms to the sea floor. Despite this, Guinness World Records 2009 listed the Ursa tension leg platform as the tallest structure in the world with a total height of . The Magnolia Tension-leg Platform in the Gulf of Mexico is even taller with a total height of . Taipei 101 in Taipei, Taiwan, set records in three of the four skyscraper categories at the time it opened in 2004; at the time the Burj Khalifa opened in 2010 it remained the world's tallest inhabited building as measured to its architectural height (spire). The height of its roof and highest occupied floor had been surpassed by the Shanghai World Financial Center with corresponding heights of . Willis Tower (formerly Sears Tower) was the highest in the final category: the greatest height to top of antenna of any building in the world at . Burj Khalifa broke the height record in all four categories for incompleted buildings. Tallest structure by category Due to the disagreements over how to measure height and classify structures, engineers have created various definitions for categories of buildings and other structures. One measure includes the absolute height of a building, another includes only spires and other permanent