About a hundred years ago, Indian and British archaeologists were on a research expedition.

They were traveling to a site along the Indus River in what's now Eastern Pakistan.

When they got there, all they saw were just giant mounds of mud, but they started digging.

So basically when they started excavating, they found a city.

Rajesh Rao has spent the last decade writing about the impact of this discovery.

They found urban planning that probably rivals modern standards in some ways,

because they found grid-like streets with houses on either sides,

and then they found amazing sanitation systems in terms of, you know,

houses having toilets and sewage systems.

Archaeologists stated these systems to 2,500 BCE, and they were way ahead of their time.

There wouldn't be extensive urban sanitation systems like this for over a thousand years.

It'd probably come down to the more recent Roman and Greek civilizations, right?

Even there, I think the kind of sophistication I would say is not there.

Archaeologists named this city Mohenjo Daro, a mound of the dead men,

and it was a lot more than just an impressive sanitation system.

It was a full square mile of immaculately constructed roads lined with brick walls,

multiple-story homes, court guards, even a public bath.

4,500 years ago, it was likely one of the biggest cities in the world.

And archaeologists eventually found more huge cities like this,

and over a thousand settlements all thought to be built by the same people,

who they call the Indus Valley civilization.

This society spanned at least 800,000 square kilometers through modern-day India,

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