How 'Trafficked' host Mariana van Zeller gains sources' trust while covering the black market

The first hurdle to covering black markets is gaining access to a world operating under the radar by design. That's no easy feat, according to Peabody Award-winning journalist Mariana van Zeller.

"It takes a lot of work. It takes months, sometimes even years of just waiting around for things to happen, knocking on doors, relentlessly pursuing people who live, work and operate in these underworlds that don't want there to be a light shone on them," van Zeller told ABC News.

Since 2020, van Zeller has been at the helm of "Trafficked with Mariana van Zeller," serving as both host and executive producer. The National Geographic documentary series explores the complex inner workings of the global underworld. Each episode follows van Zeller's investigations into a different black market, from human organs and guns to surgery and cars.

"These informal, black and gray markets, actually make up for almost half of the global economy, and yet we know so little about them. And they have an impact on our daily lives. So for me it's incredibly important to be able to witness and show what is happening in these underworlds," van Zeller said.

As the show kicked off its third season, van Zeller sat down with ABC News Live to talk about the challenges of her work and why she believes understanding is the key to help tackling these issues.

Black markets are so secretive that gaining sources' trust is an uphill battle, van Zeller said. For every "yes" there are "dozens, if not hundreds" of rejections.

"I always tell them I'm not here to judge. I do not condone what you do, but I am here to understand why you do it and to understand your motivation. Because only then we were able to sort of get an understanding and perhaps prevent these black markets from happening, from existing," van Zeller said.

The work is not without its dangers. In "Terrorist Oil," an upcoming episode that explores how illicit oil is fueling terrorist groups, van Zeller is interrogated and threatened by one man when her team shows up at crude oil refinery.

"So they at first thought that we were coming to shut their operation down and therefore leave these hundreds of families with no income. And so they weren't happy that we were there. So it took some time to sort of explain to them, look, I'm a journalist. And I kept repeating it," van Zeller said.

Extensive training and planning, staying calm and showing people respect are crucial. "I always say that no story is worth a life, obviously. So we take our safety very seriously," van Zeller said.

Van Zeller said as a woman doing this type of work comes with challenges, but there are also advantages.

"I think that as women, we are often under appreciated, or they see less of a threat when they are talking to me because they think, you know, she's weaker in some respects and therefore less threatening," van Zeller said.

She wants viewers to take away that black markets are closer than we think.

"They're actually happening all around us. They're much more prevalent than we think. They have a deep and important impact, a negative impact in our lives. So trying to understand who are the operators, what motivates them. Again, the understanding before the judging is really, truly the only way that we'll be able to tackle and combat these issues," van Zeller said.