The arrival in recent days of 15,000 Haitian immigrants at the U.S.-Mexico border near Del Rio, Texas, has been a decade in the making.

But it's these last three weeks that puzzled some: How were the migrants able to breach the U.S. border and amass so quickly?

Images of the migrants wading across the Rio Grande in recent days are a reminder of how permeable the U.S. border can be. It's also shown how the land between the river and a border wall can become a no-man's land.

Just a third of the 2,000-mile border with Mexico has some kind of barrier, and about half of those barriers are designed only to stop vehicles. People on foot can easily circumvent them.

It's not known what ultimately brought the refugees to the encampment under a bridge in Del Rio this month, but it's likely trudging across the Rio Grande and along the river's banks was not the most taxing part of their 11-year odyssey.

Live updates: Biden vows action for treatment of Haitian migrants

Although the government began using Title 42 to quickly return migrants to Mexico or their country of origin during the pandemic, it is lawful to claim asylum at or between a port of entry.