Narrator: Listen to part of a lecture in a History class.

Professor: So most cities of the ancient world tended to be small, often limited to the banks of a river.

They had very little means to expand.

These old cities couldn't really cross natural barriers like rivers,

or be located very far from water sources.

But Roman cities, on the other hand, grew much larger.

How did this happen?

Well, for one thing, the Romans had more advanced technology.

Let's look at a couple of Roman developments that allowed their cities to expand.

One development that allowed Roman cities to grow was their advanced building materials.

The Romans developed a special kind of concrete, a building material that would harden under water,

and this concrete made new kinds of structures possible.

Take their bridges, for example,

because of the special concrete, they could build better bridges,

bridges that could go across wide rivers,

bridges that were big enough to transport equipment and materials with wagons and carts.

So with these strong bridges,

Roman cities could grow on both sides of the river,

creating larger cities than would have been possible otherwise.

Another development that helped Roman cities expand was an improved way to move fresh, clean water.

People need to access to fresh water,

and the Romans created an especially effective way to bring it to them.

They built structures called "aqueducts".

Now, aqueducts are a series of open channels,

waterways that stretch from water sources high in the mountains to cities.

They were carefully planned and built so that a steady drop in altitude provided a steady flow of water to cities.

These aqueducts could move a tremendous amount of water over great distances,

and even bring fresh water to places far from rivers.

Because of this,

people could have clean water for drinking and bathing without being located near a river.

So cities were able to grow larger in new locations.