To get a feel for ancient Rome's crowded, urban conditions I headed for its seaport, a place called Ostia. 2000 years ago it was a very busy place, inhabited mostly by sailors, merchants, shippers and traders, and the kinds of businesses they frequented. Ostia was also crammed with insulae, which is Latin for apartment complexes. Now this one was called Insula Diana, and we think it was about seven to eight stories high and housed about a hundred people. Makes me wonder why, unlike any other city at the time, ancient Rome in particular was so very crowded. >> There was a law called the Lex Julia which provided citizenship for people who were living in the area around Rome, north and south. And this particular law started to bring people into the city of Rome. And pretty soon, you had hundreds of thousands of people pouring into Rome. And swelling it up to a city that, by the time of Augustus, had 1 million people in it. >> The population boom was also fed by, well, food. Outliers flocked here to Rome because they were hungry. And quite simply, the city was able to feed them. The result, single family homeowners started adding floors to their houses, four or five stories and more. No one had ever gone that high before. >> Claims are that these insulae never got above seven stories high. Now if true, the question I have as a physicist is, why not? Here's a theory. Hold on just a second, [FOREIGN]. Okay now, the ancient Romans built their apartment buildings using concrete blocks that were a lot like this dried bread. They were strong compressionally, which means you could have stacked them in principle to 20, 30, 40 stories high and they wouldn't be crushed under their own weight. But they were weak torsionally, which means that a tall building, if it were subjected to a high wind or a strong quake, they would have caused the concrete blocks to twist and bend, and cause them to snap like a twig. And that's why we think that ancient buildings, that were higher than a certain critical height, inevitably collapsed. If only the Romans had reinforced their concrete with steel rods the way we do today. Their structures could have withstood swaying and twisting forces, and the Romans could have built even higher than they did. [MUSIC] >> Well, if a building went up four or five stories and it wasn't soundly founded, it wasn't well built, there was a great risk of buildings falling down. And we know that there was constant legislation to try to prevent this. Plus, in the poorer tenements, people had to cook right there in the hallways and there was a risk of fire, and there being no easy way to escape, there was great danger.