Her ancestors fled to Mexico to escape slavery 170 years ago. She still sings in English to this day.

She is proof of how complicated migration between the United States and Mexico can be: An 86-year-old black woman, sitting in her home in northern Mexico, singing hymns in English.

Lucia Vazquez Valdez is the descendant of African Americans who fled the prospect of slavery in the United States for Mexico in the middle of the 19th century, crossing a border that barely existed. Her tribe is known as the Mascogos. After escaping their would-be slaveholders, they are still living in the tiny village their ancestors settled: Nacimiento de los Negros, or Birth of the Blacks, in the desert of Coahuila.

Valdez is the oldest living member of the tribe. She speaks almost no English. Around her, she watches as young Mascogos migrate back to the United States, this time in search of jobs on farms and ranches.

Her eyesight is gone. Her hearing is going, too. She is the matriarch of a group whose history reflects the long view on human migration, the way a community can be pushed and pulled across borders. In the process, Valdez has watched her own culture dissipate. The Mascogos have become more Mexican, more migratory and less attached to their patch of desert where droughts have made farming harder and harder.

Across the village, people ask aloud: What is left of the Mascogos? After generations of intermarriage, most of them are no longer dark-skinned. Many have forgotten how to cook the dishes their ancestors brought with them from America. They wear traditional dress only once a year on Juneteenth, the U.S. emancipation holiday. They speak Spanish, not English.

But when Valdez opens her mouth to sing, it’s like plunging into the past. She still knows the English-language hymns that her ancestors brought from the United States.

They flow through as if she’s channeling a ghost, a perfect lilting English that she can’t understand but can perfectly harmonize. Some are songs of her great grandparents and their great grandparents, who were slaves before they were unlikely Mexicans. Other songs, like “This Little Light of Mine,” found their way to the Mascogos in the 20th century. No one is exactly sure how.

Now the border north of Coahuila is the hemisphere’s sharpest line of demarcation. Valdez, her family knows, will die a Mexican citizen, not a U.S. one, a couple hours south of Texas. Almost no one else here knows the English songs.

Sometimes descendants of the Mascogos visit Valdez and ask her to sing. They record her on their iPhones to show relatives back in Texas. Her voice is proof that the tribe existed, that at least for a time the Mascogos found freedom in the desert of northern Mexico, before they packed their bags and crossed the border again.