The 600,000-Ton Petronas Towers Hang Over Hollow Ground

Two towers. 600,000 tons of concrete and steel. And under Kuala Lumpur — limestone eaten through with caves and hollow pockets. When engineers drilled test holes under an old racecourse in the middle of the city, they found something that almost stopped the world's tallest buildings before the first wall went up: a buried cliff, with hollow limestone on one side and softer rock on the other. A tower standing across it could slowly start to lean. So how do the Petronas Twin Towers stay up? The whole project was moved 61 metres. Under each tower, 104 concrete piles — some 114 metres long, deeper than a 30-storey building is tall — never rest on solid rock: they grip the ground by friction alone, under a 4.6-metre raft poured non-stop for 54 hours. Then the race began: a Japanese team and a Korean team built one tower each, a bad batch of concrete stopped the site, one tower leaned 25 mm at the 72nd floor, and a 750-ton skybridge was hung between them that still slides every time the wind blows. This is how the Petronas Towers were really built — from the caves below to the spires 452 metres up. CHAPTERS 0:00 What lies under the Petronas Towers? 2:24 A racecourse in the middle of the city 4:08 The star: César Pelli's Islamic geometry 5:52 Concrete, not steel 7:52 The buried cliff and the deepest foundations 10:49 Two countries, one race 12:15 The bad batch of concrete 13:18 25 millimetres 14:53 The bridge that slides 17:36 A stainless steel skin 18:26 The pinnacles and the world record 20:25 Life inside: double-deck elevators 21:46 Daredevils 22:36 The towers today 23:21 The answers Sources: CTBUH, Thornton Tomasetti, VSL, Motioneering, Petronas Twin Towers official information, engineering papers on the towers' high-strength concrete. Archive photos: Public Domain / CC0 (Wikimedia Commons, KITLV collection, US government). Stock footage: Pexels. Animation: Impossible Giants.