(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.
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."
Children who cross the border alone are first held in Customs and Border Protection custody, then transferred to shelters run by the Department of Health and Human Services, where they're held until they're released to sponsors in the United States.
"The vast majority of sponsors are a parent or a close family relative living in the United States," HHS says.
In other words, most of the unaccompanied minors who come to the United States already have a family member living here. And, as the Migration Policy Institute noted in a 2016 report, "the desire to reunify remains strong."
Department of Homeland Security statistics show that the vast majority of children who've come alone to the United States from Central America -- and other regions that don't neighbor the United States -- are still here.
Of the 290,000 children in this group who crossed the border without a parent since 2014, 4.3% were returned to their home countries, and 28% were granted protection by US courts.
What about the others? As of December 2020, 68% of their cases remained unresolved -- 16% had been ordered to leave, but hadn't been deported or confirmed their departure yet, and 52% of their cases were still being processed.
But it's important to note that statistics like this can be misleading, because they show a snapshot of how a large number of cases have been handled so far, but not the whole picture. Children's immigration cases can take much longer than adults' cases, due to special provisions and protections in place and an extremely backlogged court system.
There's also a significant differences in outcomes if children have legal representation. And just because a case hasn't resulted in deportation yet doesn't mean that it eventually won't.
So why are we seeing another surge of unaccompanied minors crossing the border now? There are many contributing factors at play in migrants' home countries -- and also a big change the Biden administration made.
Officials recently ended a controversial Trump administration policy that was put in place during the pandemic. That policy, which cited public health concerns, allowed the US government to kick out children who came to the border without giving them a chance to seek asylum. Critics said it flew in the face of international law and human rights norms, and endangered the lives of children seeking safety.
The Biden administration has stressed that the border isn't open, and officials have pledged to turn back most adults and families who cross. But Department of Homeland Security Secretary Alejandro Mayorkas says the cases of unaccompanied minors are different,
"They are vulnerable children," he said this week, "and we have ended the prior administration's practice of expelling them."
That means children who've crossed the border alone will have a chance to make asylum claims once again. But the outcomes could be different than previous groups faced.
The Biden administration says it's working on overhauling the system with an eye towards efficiency. If they succeed, we could see an end to lengthy waits for answers that have let many children stay in the United States for years while their cases made their way through the courts.