AMNA NAWAZ: Illegal border crossings into the United States from Mexico have dropped precipitously in the last few months.

But farther south, hundreds of thousands of migrants are still making dangerous journeys through one of the world's most treacherous jungles.

"The Atlantic"'s September issue, titled "Seventy Miles in Hell" documents migrants' efforts traveling through the Darien Gap, a once-considered-impassable region connecting Central and South America.

Staff writer at "The Atlantic" Caitlin Dickerson made several reporting trips into the jungle following migrants through the crossing.

She joins me now.

Caitlin, welcome back.

Thanks for being with us.

CAITLIN DICKERSON, "The Atlantic": Thanks so much for having me, Amna.

AMNA NAWAZ: So you and photographer Lynsey Addario made a number of trips to the Darien Gap, this deadly, nearly impenetrable jungle that hundreds of thousands of people still walk every single year.

Caitlin, for people who have never been there, will never be able to go there, just describe to us what it was like for you to make that journey, what stood out to you, what stays with you.

CAITLIN DICKERSON: The Darien Gap is this narrow strip of land that extends out of Northern Colombia into Southern Panama.

It's the only way to walk north out of South America.

It's very, very dense.

It's mountainous.

And the list of threats that migrants who make this crossing are facing is very long, everything from flash floods, which are quite common because it rains on a daily basis, to falling.

People have heart attacks from overexertion from the terrain.

There are deadly snakes.

There are jungle cats.

And on top of all of the natural risks, you also have bands of robbers who will attack migrants.

Migrants are very often robbed and, unfortunately, also frequently experience sexual assault on this journey.

So it's grueling.

I mean, that's the only way to put it.

It was certainly the hardest thing I have ever done physically.

I think that is one of things that sticks with me to this day and just the amount of desperation.

People walk into the Darien Gap and it's almost impossible to be fully prepared.

You don't know what you're going to run into.

And so you might run out of water.

You might run out of food, even if you're healthy and you're as prepared as you can possibly be, that this is an incredibly treacherous thing to try to do.

AMNA NAWAZ: And, as you include in your story, according to the United Nations, more than 800,000 people could make that same journey this year.

That would be a more than 50 percent spike over last year's numbers.

Children under the age of 5, like this young girl, this 5-year-old girl named Susej, I believe her name is, that you met along the way.

captured here by photographer Lynsey Addario, comforting her mother in this photo.

Children under 5, Caitlin, are the fastest growing group.

From spending time with these families, just tell us about them, and why are these numbers surging so dramatically now?

CAITLIN DICKERSON: I think the first thing that I heard for most of the parents who had young children with them in the Darien Gap is that they would rather be anywhere else.

So a lot of families, including Susej's -- her name Susie in English - - they had tried resettling first in Chile.

They were originally from Venezuela.

They couldn't make things work there.

And lots of places in Latin America where people might prefer to resettle, their economies were devastated by the pandemic.

And so the Darien Gap is this last resort option.

The smugglers who shepherd people into the Darien Gap do mislead people about what they're in for, make it seem like it's going to be easier.

But most people who show up at the mouth of the jungle with young kids know that they're risking their lives and know that they're risking their children's lives.

AMNA NAWAZ: We know, of course, many of those people are hoping to make it all the way to the United States.

The Biden administration, of course, put tougher border restrictions into place back in early June, severely limiting who can legally enter and claim asylum.

And that has contributed to a dramatic decline at the U.S. southern border.

We went from around 250,000 people apprehended in December to reportedly just 57,000 in July, though official numbers are not yet out.

So, Caitlin, did people you met along the way know about those restrictions?

Had word made it down to them?

CAITLIN DICKERSON: Not at all.

There's so many different factors that come into play here.

But, usually, when people are making the decision to migrate, they're not talking about U.S. policy, because, again, they feel like they're fleeing life-or-death circumstances, which is why they're willing to take risks like crossing the Darien Gap.

The Biden administration's asylum policies have had some impact, I'm sure, as has its pressure on Mexico to crack down and basically intercept people on their way to the United States.

One Biden administration policy that people in the Darien Gap did know about was the CBP One app that's being used to relieve pressure at the border so you can apply for permission to fly to the border and get an interview, rather than having to take a more dangerous option.

And it's all playing a role, but it's not enough to completely eliminate this migration because of the circumstances that people are fleeing.

And so I always caution against saying border crossings are down this month and it's all attributable to a policy that's just come down from the White House.

It's never that simple when it comes to a global issue.

AMNA NAWAZ: It does go back in some ways to this other recent piece you wrote for "The Atlantic" in which you said -- quote -- "Trying to stop migration at the border is like telling someone they can't run a marathon when they're at the finish line.

This was in a piece that was actually entitled, "There's No Such Thing as a Border Czar."

You are referencing there, of course, how Republicans have been labeling Vice President Kamala Harris.

And we should point out she, yes, was not charged with the border, per se, but she was charged with addressing root causes.

It sounds like, from your reporting, the root causes forcing people to leave have only gotten worse over time.

Is that fair?

CAITLIN DICKERSON: It is.

But the answer is a little bit more complicated, in that when the Biden administration came to office and Kamala Harris was given this position, most people crossing the southern U.S. border were coming from Central America.

And so that's where she focused her efforts.

She raised about $5.5 billion in private funds to support more jobs and improved quality of life in Central America.

And migration from Central America is going down.

But the problem is that circumstances have worsened elsewhere.

And so we have increasing numbers of people crossing the border from Venezuela, from China, from Haiti, from Ecuador, from throughout Latin America.

A single American diplomat or official can't change circumstances abroad on their own.

But I think the changing dynamic underscores just how tough this is.

You focus your efforts in one place, and then they get more difficult in another.

AMNA NAWAZ: That's Caitlin Dickerson of "The Atlantic."

Her cover story "Seventy Miles in Hell" is out now.

Caitlin, thank you so much for joining us.

Good to speak with you.

CAITLIN DICKERSON: Thank you for having me.