Apart from a few bridges that still work, the infra­struc­tur­al achieve­ments of the Roman Empire exist, for us, most­ly as ruins. With a lit­tle imag­i­na­tion, those his­toric sites give us a clear enough sense of the empire’s sheer might, but if we want to go deep­er, we should then look into the numer­ous Roman con­struc­tions that haven’t sur­vived at all. In the video below from his chan­nel Told in Stone, ancient-his­to­ry YouTu­ber Gar­rett Ryan gives his per­son­al top sev­en “lost Roman won­ders,” begin­ning with Trajan’s Bridge, whose length of more than a kilo­me­ter across the Danube made it the longest bridge ever built at that time: a project of ambi­tions befit­ting a man that his­to­ry remem­bers as one of the “Five Good Emper­ors.”

No such sta­tus for Nero, though he did com­mis­sion the Subi­a­co Dams. Nec­es­sary to cre­ate a series of arti­fi­cial lakes beneath the infa­mous ruler’s vil­la, they were the high­est dams in exis­tence until the Mid­dle Ages. Hadri­an’s more pub­lic-mind­ed white-mar­ble tem­ple at Cyz­i­cus in mod­ern-day Turkey was known as unusu­al­ly splen­did even by the stan­dards of that genre of build­ing; now only its foun­da­tions remain, with archae­o­log­i­cal digs turn­ing up the occa­sion­al frag­ment.

In the for­mer Con­stan­tino­ple, the Church of the Holy Apos­tles is remem­bered as the sec­ond-most famous van­ished ear­ly Chris­t­ian church, after only the old St. Peter’s Basil­i­ca in Rome. Hagia Sophia may still be around, but the Col­umn of Jus­tin­ian that once stood between it and the Great Palace was melt­ed down for scrap long ago.

We only know of these lost struc­tures because we have his­tor­i­cal attes­ta­tions of their exis­tence, most vivid­ly in paint­ings: take the Pyra­mid of Romu­lus, a strik­ing tomb that appears near the Vat­i­can in The Vision of the Cross, which Raphael’s assis­tants pro­duced after his death. Oth­ers, like the Colos­sus of Nero from which the Colos­se­um takes its name, are rep­re­sent­ed on coins. In the video at the top, Ryan cov­ers three more “for­got­ten Roman megapro­jects”: Claudius’ tun­nel of record-mak­ing length through the moun­tain between the Riv­er Liris and the Fucine Lake, Tra­jan’s mil­i­tary high­way through the “Iron Gates” of the Danube, and Octa­vian’s secret har­bor at Lake Aver­nus. In our time, of course, there are no more emper­ors, Roman or oth­er­wise, and we’re sure­ly bet­ter off for it. But we can still admire — to use a twen­ty-first cen­tu­ry con­cept — the state capac­i­ty they com­mand­ed.

Relat­ed con­tent:

The Advanced Tech­nol­o­gy of Ancient Rome: Auto­mat­ic Doors, Water Clocks, Vend­ing Machines & More

How the Ancient Romans Built Their Roads, the Life­lines of Their Vast Empire

The Genius Engi­neer­ing of Roman Aque­ducts

The Roman Colos­se­um Decon­struct­ed: 3D Ani­ma­tion Reveals the Hid­den Tech­nol­o­gy That Pow­ered Rome’s Great Are­na

The Amaz­ing Engi­neer­ing of Roman Baths

Built to Last: How Ancient Roman Bridges Can Still With­stand the Weight of Mod­ern Cars & Trucks

Based in Seoul, Col­in Marshall writes and broad­casts on cities, lan­guage, and cul­ture. He’s the author of the newslet­ter Books on Cities as well as the books 한국 요약 금지 (No Sum­ma­riz­ing Korea) and Kore­an Newtro. Fol­low him on the social net­work for­mer­ly known as Twit­ter at @colinmarshall.

The play­wright Tris­tan Bernard is said to have eat­en lunch at the Eif­fel Tow­er every day, but not because he liked the menu in its café: rather, because it was the only place in Paris with no view of the Eif­fel Tow­er. His view wasn’t whol­ly eccen­tric in the decades after its con­struc­tion, in the late eigh­teen-eight­ies, when the struc­ture had yet to become the most beloved in France, and per­haps in the world. Yet not far behind the Eif­fel Tow­er as a must-vis­it tourist attrac­tion in a town full of them is Paris’ least beloved build­ing: the Tour Mont­par­nasse, which since its com­ple­tion in 1973 has stood in infamy as the only sky­scraper in the cen­ter of the city.

Unlike the Eif­fel Tow­er, which was com­mis­sioned in part to cel­e­brate the cen­ten­ni­al of the French Rev­o­lu­tion, the Tour Mont­par­nasse projects no polit­i­cal sym­bol­ism; unlike Notre-Dame de Paris, or Sacré-Cœur de Mont­martre, it has no reli­gious sig­nif­i­cance. Its pur­pose is whol­ly com­mer­cial, befit­ting a large office build­ing with a shop­ping mall — or now, the remains of a shop­ping mall — at the bot­tom. But when it was first con­ceived in 1958, it embod­ied the very image of moder­ni­ty in a built envi­ron­ment that was dilap­i­dat­ed where it wasn’t war-torn. A mod­ern sky­scraper would show the world, unmis­tak­ably, that Paris had stepped ful­ly into the twen­ti­eth cen­tu­ry of indoor plumb­ing, elec­tric­i­ty, fast trains, and telecom­mu­ni­ca­tion.

This mis­sion gained the full back­ing of none oth­er than Andre Mal­raux, then France’s first Min­is­ter of Cul­tur­al Affairs. Unfor­tu­nate­ly, nine­teen-fifties Europe lacked the tech­nol­o­gy, exper­tise, and mon­ey required for a 60-sto­ry sky­scraper, let alone one serv­ing as the cen­ter­piece of a sweep­ing rede­vel­op­ment project that includ­ed gleam­ing new res­i­den­tial blocks and a com­plete­ly rebuilt Mont­par­nasse Sta­tion. The tow­er could­n’t even break ground until 1969, by which time the build­ing’s once-cut­ting-edge mid-cen­tu­ry design — hard­ly a uni­ver­sal hit even in maque­tte form — had already begun to look passé. (Part of the prob­lem was sure­ly its col­or, which archi­tect Philippe Tré­ti­ack described as hav­ing “a touch of the nico­tine stain about it.”)

When the Tour Mont­par­nasse turned 50 a few years ago, I hap­pened to be in Paris on my hon­ey­moon. Noth­ing was hap­pen­ing to mark the occa­sion, apart from the long-ongo­ing dis­cus­sions about whether to ren­o­vate the thing or just knock it down. The for­mer option hav­ing won the day, you can see the details of the planned extreme makeover in the B1M video above. Rather than destroy­ing the exist­ing build­ing, the idea is to do the next best thing and make it invis­i­ble. This ambi­tious project will install a new façade of clear glass and bands of sky gar­dens, among oth­er changes, in order to light­en its bur­den­some visu­al mass. But how­ev­er rad­i­cal its trans­for­ma­tion, one sus­pects that it will remain most appre­ci­at­ed as the only place in Paris with­out a view of the Tour Mont­par­nasse.

Relat­ed con­tent:

How Paris Became Paris: The Sto­ry Behind Its Icon­ic Squares, Bridges, Mon­u­ments & Boule­vards

Watch the Build­ing of the Eif­fel Tow­er in Time­lapse Ani­ma­tion

The Archi­tec­tur­al His­to­ry of the Lou­vre: 800 Years in Three Min­utes

The Cre­ation & Restora­tion of Notre-Dame Cathe­dral, Ani­mat­ed

Why Europe Has So Few Sky­scrap­ers

Why Do Peo­ple Hate Mod­ern Archi­tec­ture?: A Video Essay

Based in Seoul, Col­in Marshall writes and broad­casts on cities, lan­guage, and cul­ture. He’s the author of the newslet­ter Books on Cities as well as the books 한국 요약 금지 (No Sum­ma­riz­ing Korea) and Kore­an Newtro. Fol­low him on the social net­work for­mer­ly known as Twit­ter at @colinmarshall.

No one who trav­els to Flo­rence can help see­ing the dome of the Cathe­dral of Saint Mary of the Flower. That’s true not just because of its sheer loom­ing phys­i­cal pres­ence over the rest of the city, but also because of its impor­tance as an achieve­ment in var­i­ous kinds of his­to­ry, from that of engi­neer­ing to archi­tec­ture to reli­gion. Its sto­ry is told by art his­to­ri­ans Beth Har­ris and Steven Zuck­er in their new Smarthis­to­ry video above, which begins in the year 1417. At the time, Zuck­er explains, Flo­rence had a “huge” prob­lem: the ground­work for its ambi­tious­ly large cathe­dral had been laid a cen­tu­ry before, but nobody knew how to build the dome for which its plans called.

The assump­tion, says Har­ris, was that “by the time they had to build it, they would fig­ure out how to do it,” a reflec­tion of both the more relaxed speed of con­struc­tion in the fif­teenth cen­tu­ry, as well as a pace of inno­va­tion that must have felt rapid­ly on the increase.

Such a struc­ture had­n’t been built since the Pan­theon in antiq­ui­ty, the out­do­ing of which would, at least in the­o­ry, con­firm Florence’s recep­tion of the torch of civ­i­liza­tion from Rome. But none of the tra­di­tion­al tech­niques could sup­port a dome of this size, atop so high a tow­er, dur­ing con­struc­tion. Sal­va­tion even­tu­al­ly came in the unpromis­ing form of Fil­ip­po Brunelleschi, an archi­tect, sculp­tor, and gold­smith with­out much of a résumé — but, cru­cial­ly, with a deep under­stand­ing of the Pan­theon.

“Brunelleschi real­ized that hemi­spher­i­cal domes func­tion in a self-sup­port­ing man­ner if they’re con­struct­ed out of self-sup­port­ing con­cen­tric cir­cles,” Zuck­er says, and his chal­lenge was to use that knowl­edge to build an octag­o­nal dome. This involved design­ing two domes, a thick inner one cov­ered by a thin out­er one. Drop €30 on a tick­et, and you can ascend the stairs through the inter-dome gap your­self. There the walls reveal the her­ring­bone brick pat­tern that kept the struc­ture sta­ble; at a larg­er scale, those bricks form struc­tur­al ele­ments, much like over­sized ver­sions of the stones used to build arch­es since time immemo­r­i­al. Regard­ing almost any pic­ture of Flo­rence, your eye may go straight to the cathe­dral, drawn both to the dome and to the splen­dor of its oth­er era-mix­ing archi­tec­tur­al fea­tures. But only from the inside can you under­stand how it all works.

Relat­ed con­tent:

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

How Fil­ip­po Brunelleschi, Untrained in Archi­tec­ture or Engi­neer­ing, Built the World’s Largest Dome at the Dawn of the Renais­sance

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

Why Hasn’t the Pantheon’s Dome Col­lapsed?: How the Romans Engi­neered the Dome to Last 19 Cen­turies and Count­ing

How Design­ing Build­ings Upside-Down Rev­o­lu­tion­ized Archi­tec­ture, Mak­ing Pos­si­ble St. Paul’s Cathe­dral, Sagra­da Família & More

His­to­ri­an Answers Burn­ing Ques­tions About The Renais­sance

Based in Seoul, Col­in Marshall writes and broad­casts on cities, lan­guage, and cul­ture. He’s the author of the newslet­ter Books on Cities as well as the books 한국 요약 금지 (No Sum­ma­riz­ing Korea) and Kore­an Newtro. Fol­low him on the social net­work for­mer­ly known as Twit­ter at @colinmarshall.

We know more or less every­thing we could pos­si­bly know about ancient Egypt­ian civ­i­liza­tion. That owes in large part to the advanced state of record-keep­ing it achieved, and how many of its writ­ings have sur­vived, up to and includ­ing — as pre­vi­ous­ly fea­tured here on Open Cul­ture — a home­work assign­ment and a list of excus­es giv­en by builders who missed work. There just hap­pens to be one espe­cial­ly glar­ing gap in our knowl­edge: exact­ly how the ancient Egyp­tians built the Pyra­mids of Giza. This inter­sec­tion of rel­a­tive igno­rance and extreme fas­ci­na­tion has, as archi­tec­ture YouTu­ber Dami Lee acknowl­edges in the video above, inspired no end of crack­pot-ism. Noth­ing could be as unpromis­ing as unso­licit­ed con­tact from some­one claim­ing to have dis­cov­ered the secret of the pyra­mids.

The case of a Kore­an inde­pen­dent researcher called Huni Choi proved to be dif­fer­ent, for rea­sons Lee uses the video to lay out. Con­ven­tion­al assump­tions about how the pyra­mids were built hold that work­ers would have had to drag the stones up one or more ramps, though the dimen­sions of the struc­tures dic­tate that the project would neces­si­tate huge, com­plex, or huge and com­plex ramp sys­tems — whose own con­struc­tion has some­how left behind not a trace of evi­dence.

Accord­ing to Choi, “the Great Pyra­mid was­n’t built on its own, but through a chain of ‘sac­ri­fi­cial’ struc­tures” designed to be “can­ni­bal­ized.” The idea is that the pyra­mids were “over­built,” start­ing with a gigan­tic “trape­zoidal mass” with an inte­grat­ed ramp sys­tem, which, after being topped out, was then carved down into the pyra­mid shape we still find so famil­iar and com­pelling.

If true, Choi’s the­o­ry would solve the long-intractable prob­lem of the point­ed tops, which posed such a thorny engi­neer­ing prob­lem that even oth­er pyra­mid-build­ing civ­i­liza­tions seem­ing­ly avoid­ed even attempt­ing them. It also accounts for how the Egypt­ian design­ers and builders could have kept an eye on the angles all the while, in order to make sure the things were going up straight. And what of the left­over stone cut away from each pyra­mid? Why, it would sim­ply have been re-used for the con­struc­tion of the next one. This all squares not just with the esti­mat­ed mass of the Giza com­plex, but also with appar­ent ancient Egypt­ian atti­tudes toward the nat­ur­al and built envi­ron­ment. Alas, unlike in, say, physics, an archae­o­log­i­cal the­o­ry like this one remains dif­fi­cult to prove dis­pos­i­tive­ly, bar­ring anoth­er tech­no­log­i­cal break­through that enables a new form of analy­sis of the pyra­mids them­selves. Still, it’s a lot more sat­is­fy­ing than just assum­ing some ancient aliens did it.

Relat­ed con­tent:

Pyra­mids of Giza: Ancient Egypt­ian Art and Archaeology–a Free Online Course from Har­vard

A Walk­ing Tour Around the Pyra­mids of Giza: 2 Hours in Hi Def

Who Real­ly Built the Egypt­ian Pyra­mids — and How Did They Do It?

How Did They Build the Great Pyra­mid of Giza?: An Ani­mat­ed Intro­duc­tion

Ancient Egypt­ian Pyra­mids May Have Been Built with Water: A New Study Explore the Use of Hydraulic Lifts

Isaac New­ton The­o­rized That the Egypt­ian Pyra­mids Revealed the Tim­ing of the Apoc­a­lypse: See His Burnt Man­u­script from the 1680s

Based in Seoul, Col­in Marshall writes and broad­casts on cities, lan­guage, and cul­ture. He’s the author of the newslet­ter Books on Cities as well as the books 한국 요약 금지 (No Sum­ma­riz­ing Korea) and Kore­an Newtro. Fol­low him on the social net­work for­mer­ly known as Twit­ter at @colinmarshall.