Why the Leaning Tower of Pisa Still Hasn’t Fallen Over, Even After 650 Years

The Lean­ing Tow­er of Pisa has stood, in its dis­tinc­tive fash­ion, for six and a half cen­turies now. But it has­n’t always leaned at the same angle: to get the most dra­mat­ic view, the best time to go see it was the ear­ly nine­teen-nineties, when its tilt had reached a full 5.5 degrees. Grant­ed, at that point — when by some reck­on­ings, the tow­er should no longer have been stand­ing at all — it was closed to the pub­lic, pre­sum­ably due to fears that the sheer weight of tourism would push it over the tip­ping point. The 1989 col­lapse of Pavi­a’s eleventh-cen­tu­ry Civic Tow­er also had some­thing to do with it: could­n’t some­thing be done to spare Pisa’s world-famous land­mark from a sim­i­lar fate?

Attempts to shore up the Lean­ing Tow­er up to that point had a check­ered his­to­ry, to put it mild­ly. Built on soft soil, it start­ed to lean in back in the twelfth cen­tu­ry, before its con­struc­tion was even com­plete. The process of that con­struc­tion, in the event, took near­ly 200 years to com­plete; dur­ing one decades-long pause dur­ing a par­tic­u­lar­ly embat­tled peri­od for the Repub­lic of Pisa, the tow­er actu­al­ly set­tled enough to pre­vent its lat­er col­lapse, though it remained aslant. In the late thir­teenth cen­tu­ry, the best solu­tion avail­able for this con­di­tion was sim­ply to build the rest of its floors in a curved shape in com­pen­sa­tion.

For cen­turies after, the sight of the Lean­ing Tow­er tempt­ed gen­er­a­tions of struc­tur­al engi­neers to straight­en it out. It even tempt­ed non-engi­neers like Ben­i­to Mus­soli­ni, who in 1934 ordered large amounts of con­crete pumped into its foun­da­tion. Like most such oper­a­tions, it only made the tow­er lean more; only in the sec­ond half of the twen­ti­eth cen­tu­ry did the tech­nol­o­gy come along to ana­lyze its foun­da­tions and the soil in which they were embed­ded clear­ly enough to devise an effec­tive solu­tion. This end­ed up involv­ing the removal of soil with a slant­ed drill from under the tow­er’s high­er end, which even­tu­al­ly brought it back to lean about four degrees, as it did near­ly two cen­turies ago. After sub­se­quent sta­bi­liza­tion work, it was guar­an­teed to remain upright for at least anoth­er two cen­turies.

You can learn more about the con­struc­tion and re-engi­neer­ing of the Lean­ing Tow­er in the videos above from TED-Ed and Dis­cov­ery UK. But you may still ask, why was it nev­er brought down by an earth­quake? “It turns out that the squishy soil at the structure’s base that caused its fetch­ing infir­mi­ty – the tow­er was tilt­ing by the time its sec­ond sto­ry was built in 1178 – con­tains the secret to its struc­tur­al resilience,” writes Joe Quirke at Glob­al Con­struc­tion Review. This means that “the soft­ness of the foun­da­tion soil cush­ions the tow­er from vibra­tions in such a way that the tow­er does not res­onate with earth­quake ground motion.” The very ele­ment that caused the tow­er to lean kept it from falling over, an irony to match the fact that such a seem­ing­ly mis­be­got­ten build­ing project has become one of Italy’s proud­est tourist attrac­tions.

Relat­ed con­tent:

See Galileo’s Famous Grav­i­ty Exper­i­ment Per­formed in the World’s Largest Vac­u­um Cham­ber, and on the Moon

When the Indi­ana Bell Build­ing Was Rotat­ed 90° While Every­one Worked Inside in 1930 (by Kurt Vonnegut’s Archi­tect Dad)

High-Res­o­lu­tion Walk­ing Tours of Italy’s Most His­toric Places: The Colos­se­um, Pom­peii, St. Peter’s Basil­i­ca & More

How the World’s Biggest Dome Was Built: The Sto­ry of Fil­ip­po Brunelleschi and the Duo­mo in Flo­rence

Why Hiroshi­ma, Despite Being Hit with the Atom­ic Bomb, Isn’t a Nuclear Waste­land Today

Based in Seoul, Col­in Marshall writes and broad­casts on cities, lan­guage, and cul­ture. His projects include the Sub­stack newslet­ter Books on Cities, the book The State­less City: a Walk through 21st-Cen­tu­ry Los Ange­les and the video series The City in Cin­e­ma. Fol­low him on Twit­ter at @colinmarshall or on Face­book.