The Underground Railroad ran south as well as north. For enslaved people in Texas, refuge in Canada must have seemed impossibly far away. Fortunately, slavery was also illegal in Mexico.

Researchers estimate 5,000 to 10,000 people escaped from bondage into Mexico, says Maria Hammack, who is writing her dissertation about this topic at the University of Texas at Austin. But she thinks the actual number could be even higher.

“These were clandestine routes and if you got caught you would be killed and lynched, so most people didn’t leave a lot of records,” says Hammack.

There’s some evidence that tejanos, or Mexicans in Texas, acted as “conductors” on the southern route by helping people get to Mexico. In addition, Hammack has also identified a Black woman and two white men who helped enslaved workers escape and tried to find a home for them in Mexico.

Mexico abolished slavery in 1829 when Texas was still part of the country, in part prompting white, slave-holding immigrants to fight for independence in the Texas Revolution. Once they formed the Republic of Texas in 1836, they made slavery legal again, and it continued to be legal when Texas joined the U.S. as a state in 1845.

Enslaved people in Texas were aware that there was a country to the south where they could find different levels of freedom (though indentured debt servitude existed in Mexico, it was not the same as chattel slavery). Hammack has discovered one runaway named Tom who had been enslaved by Sam Houston. Houston was a president of the Republic of Texas who’d fought in the Texas Revolution. Once Tom got across the border, he joined the Mexican military that Houston had fought against.

Fugitive enslaved people got to Mexico in many different ways. Some went on foot, while others rode horses or snuck aboard ferries bound for Mexican ports. Stories spread about enslaved people who crossed the Rio Grande river dividing Texas from Mexico by floating on bales of cotton, and several Texas newspapers reported in July 1863 that three enslaved people had escaped this way. Even if this wasn’t logistically possible, the imagery of floating to freedom on a symbol of slavery was strong.

But it wasn’t only enslaved people in Texas who found freedom in Mexico. “I have found individuals who made it all the way from North Carolina, Mississippi, Louisiana, Alabama,” Hammack says.

Slaveholders knew that enslaved people were escaping to Mexico, and the U.S. tried to get Mexico to sign a fugitive slave treaty. Just as the Fugitive Slave Act of 1850 had compelled free states to return escapees to the south, the U.S. wanted Mexico to return escaped enslaved people to the U.S. But Mexico refused to sign such a treaty, insisting that all enslaved people were free when they set foot on Mexican soil. Despite this, some U.S. owners of enslaved people still hired slave catchers to illegally kidnap escapees in Mexico.

It’s unclear how organized the southern “underground railroad” was. Hammack says some enslaved people may have found their way to Mexico without assistance. Other evidence suggests tejanos, especially poor tejanos, played a part in helping escapees get to Mexico.

Hammack and researcher Roseann Bacha-Garza have also identified a mixed-race family from Alabama who moved to southern Texas near the Rio Grande and helped enslaved people escape to Mexico. The wife, Matilda Hicks, was a formerly enslaved woman. Her husband, Nathaniel Jackson, was the son of the man whose plantation she used to work on.

In addition, some northern abolitionists traveled south to help enslaved people reach Mexico.

“I have come across abolitionists from the north who were going to Mexico to petition Mexico to allow them to buy land to establish colonies for runaway slaves and free blacks,” Hammack says. In the early 1830s, Quaker abolitionist Benjamin Lundy “was actively petitioning the Mexican government to allow for colonies to be established for, I guess what we would consider now, refugees.”

Lundy’s plan to start a free colony in Mexico’s Texas region was thwarted when it separated from Mexico and legalized slavery. Later, in 1852, Seminole groups that included runaway enslaved people successfully petitioned the Mexican government for land. “It still belongs to their descendants and they still live there to this day in Mexico,” Hammack says.

These and other refugees fleeing slavery through the southern “underground railroad” all benefited from Mexico’s willingness to give them a safe haven.