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.

In Rome, one does­n’t have to look ter­ri­bly hard to find ancient build­ings. But even in the Eter­nal City, not all ancient build­ings have come down to us in equal­ly good shape, and prac­ti­cal­ly none of them have held up as well as the Pan­theon. Once a Roman tem­ple and now a Catholic church (as well as a for­mi­da­ble tourist attrac­tion), it gives its vis­i­tors the clear­est and most direct sense pos­si­ble of the majesty of antiq­ui­ty. But how has it man­aged to remain intact for nine­teen cen­turies and count­ing when so much else in ancient Rome’s built envi­ron­ment has been lost? Ancient-his­to­ry Youtu­ber Gar­rett Ryan explains that in the video above.

“Any answer has to begin with con­crete,” Ryan says, the Roman vari­ety of which “cured incred­i­bly hard, even under­wa­ter. Sea water, in fact, made it stronger.” Its strength “enabled the cre­ation of vaults and domes that rev­o­lu­tion­ized archi­tec­ture,” not least the still-sub­lime dome of the Pan­theon itself.

Anoth­er impor­tant fac­tor is the Roman bricks, “more like thick tiles than mod­ern rec­tan­gu­lar bricks,” used to con­struct the arch­es in its walls. These “helped to direct the gar­gan­tu­an weight of the rotun­da toward the mason­ry ‘piers’ between the recess­es. And since the arch­es, made almost entire­ly of brick, set much more quick­ly than the con­crete fill in which they were embed­ded, they stiff­ened the struc­ture as it rose.”

This has­n’t kept the Pan­theon’s floor from sink­ing, cracks from open­ing in its walls, but such com­par­a­tive­ly minor defects could hard­ly dis­tract from the spec­ta­cle of the dome (a feat not equaled until Fil­ip­po Brunelleschi came along about 1300 years lat­er). “The archi­tect of the Pan­theon man­aged hor­i­zon­tal thrust — that is, pre­vent­ed the dome from spread­ing or push­ing out the build­ing beneath it – by mak­ing the wall of the rotun­da extreme­ly thick and embed­ding the low­er third of the dome in their mass.” Even the ocu­lus at the very top strength­ens it, “both by obvi­at­ing the need for a struc­tural­ly dan­ger­ous crown and through its mason­ry rim, which func­tioned like the key­stone of an arch.” We may no longer pay trib­ute to the gods or emper­ors to whom it was first ded­i­cat­ed, but as an object of archi­tec­tur­al wor­ship, the Pan­theon will sure­ly out­last many gen­er­a­tions to come.

Relat­ed con­tent:

The Beau­ty & Inge­nu­ity of the Pan­theon, Ancient Rome’s Best-Pre­served Mon­u­ment: An Intro­duc­tion

How to Make Roman Con­crete, One of Human Civilization’s Longest-Last­ing Build­ing Mate­ri­als

A Street Musi­cian Plays Pink Floyd’s “Time” in Front of the 1,900-Year-Old Pan­theon in Rome

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

The Mys­tery Final­ly Solved: Why Has Roman Con­crete Been So Durable?

Build­ing The Colos­se­um: The Icon of Rome

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.