(CNN)The statistics are staggering. More than 400,000 migrant children have crossed the US border without their parents since 2003.
And each time a new wave arrives, political controversy follows.
The numbers are on the rise again, with some children arriving who are as young or 6 or 7. This increase is sparking fierce debate in Washington, concern from children's advocates and an emergency response from the Biden administration.
Why have so many kids made this dangerous journey? And what happens to them once they reach the United States?
Here are some of the key things we know.
They're fleeing desperate conditions
There are many different reasons migrant children travel alone to the United States. CNN's years of reporting at the border and conversations with experts reveal a common thread: It's not a decision any family makes lightly.
Many of these children, who the government dubs "unaccompanied minors," make asylum claims when they arrive because they're fleeing persecution, gang violence and other forms of organized crime. Dire economic circumstances in their home countries may also contribute to their decisions to leave.
Some parents initially make the journey with their children, buoyed by misleading statements smugglers use to entice them on the trek. But families sometimes find themselves making different decisions once they reach northern Mexico and come to understand the realities of the border.
In 2019, for example, some parents started sending children alone across the border once they realized the US government was sending families back to Mexico but not kids traveling alone.
And those harrowing decisions are happening again, Hope Border Institute Deputy Director Marisa Limón Garza told CNN this week.
"This comes with great sacrifice. I don't think it's lost on any of these parents," she said. "This is a grim choice."