This is 99% invisible. I'm Roman Mars.

It's hard to overstate the vastness of the Skid Row neighborhood in L.A.

It spans roughly 50 blocks, which is about one fifth the entire downtown area of Los Angeles.

In some ways, the streets of Skid Row are a bit like other parts of L.A.

That's reporter Carla Greene.

There are corner stores in street vendors selling everything from jewelry to loose cigarettes.

Their old brick townhouses filled with low-income housing.

There are parks where grown-ups gather to talk and children gather to play.

But it's also very clear when you've entered Skid Row.

For one thing, there's the smell.

The streets of Skid Row often stink of urine and other excrement,

baking under the hot Los Angeles sun.

The gutters are lined with trash.

And the sidewalks are mostly and on some blocks entirely,

occupied by people's makeshift homes.

A dizzying array of tarps and tents stretch out for blocks and blocks.

One structure jutting up against the next and the next.

And there are lots of people on the street.

Many of them pushing special red shopping carts.

The carts are handed out by a local advocacy group,

because the police used to arrest people for having what they believed were stolen shopping carts.

And so a lot of people got to do is just be homeless and come here,

...