1 00:00:08,041 --> 00:00:12,717 In 1990, the Italian government enlisted top engineers 2 00:00:12,717 --> 00:00:16,717 to stabilize Pisa’s famous Leaning Tower. 3 00:00:16,717 --> 00:00:20,789 There’d been many attempts to right the tower during its 800 year history, 4 00:00:20,789 --> 00:00:25,923 but this team’s computer models revealed the urgency of their situation. 5 00:00:25,923 --> 00:00:31,592 They projected the tower would topple if it reached an angle of 5.44 degrees— 6 00:00:31,592 --> 00:00:35,132 and it was currently leaning at 5.5. 7 00:00:35,132 --> 00:00:39,857 No one knew how the tower was still standing, but the crisis was clear: 8 00:00:39,857 --> 00:00:43,987 they had to solve a problem that stumped centuries of engineers, 9 00:00:43,987 --> 00:00:46,462 and they needed to do it fast. 10 00:00:46,462 --> 00:00:48,102 To understand their situation, 11 00:00:48,102 --> 00:00:51,832 it’s helpful to understand why the tower tilted in the first place. 12 00:00:51,832 --> 00:00:55,806 In the 12th century, the wealthy maritime republic of Pisa 13 00:00:55,806 --> 00:01:00,663 set about turning its cathedral square into a magnificent landmark. 14 00:01:00,663 --> 00:01:04,143 Workers embellished and enlarged the existing church, 15 00:01:04,143 --> 00:01:08,096 and added a massive domed baptistry to the plaza. 16 00:01:08,096 --> 00:01:15,285 In 1173, construction began on a free-standing campanile, or bell tower. 17 00:01:15,285 --> 00:01:19,265 The engineers and architects of the time were masters of their craft. 18 00:01:19,265 --> 00:01:21,423 But for all their engineering knowledge, 19 00:01:21,423 --> 00:01:25,337 they knew far less about the ground they stood on. 20 00:01:25,337 --> 00:01:29,227 Pisa’s name comes from a Greek word for “marshy land," 21 00:01:29,227 --> 00:01:35,336 which perfectly describes the clay, mud, and wet sand below the city’s surface. 22 00:01:35,336 --> 00:01:39,911 Ancient Romans counteracted similar conditions with massive stone pillars 23 00:01:39,911 --> 00:01:44,161 called piles which rest on Earth’s stable bedrock. 24 00:01:44,161 --> 00:01:49,485 However, the tower’s architects believed a three-meter foundation would suffice 25 00:01:49,485 --> 00:01:52,005 for their relatively short structure. 26 00:01:52,005 --> 00:01:55,451 Unfortunately for them, less than five years later, 27 00:01:55,451 --> 00:01:59,454 the tower’s southern side was already underground. 28 00:01:59,454 --> 00:02:03,672 Such a shifting foundation would normally have been a fatal flaw. 29 00:02:03,672 --> 00:02:05,440 If workers added more weight, 30 00:02:05,440 --> 00:02:08,687 the pressure from upper stories would sink the structure 31 00:02:08,687 --> 00:02:11,223 and fatally increase the lean. 32 00:02:11,223 --> 00:02:15,334 But construction halted at the fourth story for nearly a century 33 00:02:15,334 --> 00:02:18,951 as Pisa descended into prolonged warfare. 34 00:02:18,951 --> 00:02:22,111 This long pause allowed the soil to settle, 35 00:02:22,111 --> 00:02:25,574 and when construction began again in 1272, 36 00:02:25,574 --> 00:02:29,156 the foundation was on slightly more stable footing. 37 00:02:29,156 --> 00:02:32,708 Under the direction of architect Giovanni di Simone, 38 00:02:32,708 --> 00:02:35,694 workers compensated for the tower’s minor tilt 39 00:02:35,694 --> 00:02:40,213 by making the next few floors taller on the southern side. 40 00:02:40,213 --> 00:02:44,798 But the weight of the extra masonry made that side sink even deeper. 41 00:02:44,798 --> 00:02:48,138 By the time they completed the seventh floor and bell chamber, 42 00:02:48,138 --> 00:02:52,263 the angle of the tilt was 1.6 degrees. 43 00:02:52,263 --> 00:02:56,776 For centuries, engineers tried numerous strategies to address the lean. 44 00:02:56,776 --> 00:03:01,987 In 1838, they dug a walkway around the base to examine the sunken foundation. 45 00:03:01,987 --> 00:03:06,047 But removing the supporting sand only worsened the tilt. 46 00:03:06,047 --> 00:03:12,520 In 1935, the Italian Corps of Engineers injected mortar to strengthen the base. 47 00:03:12,520 --> 00:03:16,500 However, the mortar wasn’t evenly distributed throughout the foundation, 48 00:03:16,500 --> 00:03:19,612 resulting in another sudden drop. 49 00:03:19,612 --> 00:03:23,607 All these failed attempts, along with the ever-sinking foundation, 50 00:03:23,607 --> 00:03:26,695 moved the tower closer to its tipping point. 51 00:03:26,695 --> 00:03:29,853 And without definitive knowledge of the soil composition, 52 00:03:29,853 --> 00:03:33,356 engineers couldn’t pinpoint the tower’s fatal angle 53 00:03:33,356 --> 00:03:36,533 or devise a way to stop its fall. 54 00:03:36,533 --> 00:03:38,795 In the years following WWII, 55 00:03:38,795 --> 00:03:42,902 researchers developed tests to identify those missing variables. 56 00:03:42,902 --> 00:03:48,925 And in the 1970’s, engineers calculated the curved tower’s center of gravity. 57 00:03:48,925 --> 00:03:51,715 With this data and new computing technology, 58 00:03:51,715 --> 00:03:56,417 engineers could model how stiff the soil was, the tower’s trajectory, 59 00:03:56,417 --> 00:04:01,556 and the exact amount of excavation needed for the tower to remain standing. 60 00:04:01,556 --> 00:04:05,236 In 1992, the team drilled diagonal tunnels 61 00:04:05,236 --> 00:04:10,565 to remove 38 cubic meters of soil from under the tower’s north end. 62 00:04:10,565 --> 00:04:16,551 Then, they temporarily counterbalanced the structure with 600 tons of lead ingots 63 00:04:16,551 --> 00:04:19,811 before anchoring the base with steel cables. 64 00:04:19,811 --> 00:04:22,605 More than six centuries after its construction, 65 00:04:22,605 --> 00:04:27,852 the tower was finally straightened… to a tilt of about four degrees. 66 00:04:27,852 --> 00:04:29,872 No one wanted the tower to fall, 67 00:04:29,872 --> 00:04:33,988 but they also didn’t want to lose the landmark’s most famous feature. 68 00:04:33,988 --> 00:04:38,839 Today the tower stands at 55– or 56– meters tall, 69 00:04:38,839 --> 00:04:42,309 and it should remain stable for at least 300 years 70 00:04:42,309 --> 00:04:45,793 as a monument to the beauty of imperfection.