This town in Texas "ended up" south of the Rio Grande in Mexico, when a local irrigation company diverted the Rio Grande in the 1900s. A geography professor discovered this 60 years later and the residents of this town were labelled the "lost Americans", as the land was still US territory. The US government formally ceded the land (a bit over 400 acres) to Mexico in the early 1970s. What's the name of the (now) Mexican town?

Rio Rico, a dusty village on the Texas-Mexico border just 50 miles from the Gulf of Mexico might seem like an odd flashpoint for an international incident. But in 1906, an American irrigation company, afraid that flooding might cut off access to one of its pumping plants, dug a new channel for the Rio Grande, cutting off a switchback in the river's course. This moved 413 acres of American territory south of the river, and set off a series of border disputes that lasted almost 80 years.

Lots of legal wrangling ensued, and finally an appeals court granted U.S. citizenship to the Mexicans who had settled the area. Most immediately left the now-sleepy town for more hospitable parts of their new homeland, and the United States handed over the land to Mexico in 1977. To this day, Rio Rico is the last piece of U.S. soil ever ceded to a foreign power.

Texas State Historical Association
RIO RICO, TX. Río Rico, located on the Rio Grande in Hidalgo County, was separated from Texas by a "meander cut" in violation of the treaty between Mexico and the United States. The community was originally located north of the Rio Grande on a narrow finger of land surrounded by an S-shaped curve in the river. In July 1906, however, the American Rio Grande Land and Irrigation Company constructed a cutoff to force the river into a straighter channel. As a result, the 413 acres on which Río Rico is situated came to be located south of the river. The company was eventually taken to court and fined for the diversion of the river, but neither country resolved the issue of territorial rights. The United States never formally relinquished title to the land, since international law dictates that only natural changes of a river's course can transfer territory; over time, however, Río Rico came to be administered by the Mexican government. The issue of territorial rights drew notice once again in 1967, when James Hill, Jr., a geography professor at Arizona State University, rediscovered the shift while studying old geological survey maps. The United States Boundary Commission and the State Department investigated and confirmed Hill's findings, and in 1972 the United States officially ceded the tract of land to the Republic of Mexico. In 1972, however, a resident of Río Rico, Homero Cantú Treviño, filed suit to prevent the United States Immigration and Naturalization Service from deporting him from Texas to Mexico, arguing that he was an American citizen. In 1976 a judge ruled against him; the next year an appeals court ruled that because the 1906 cutoff was unauthorized, anyone born in Río Rico from 1906 to 1972 was entitled to United States citizenship. The Río Rican acreage is known in Mexico as the Horcón tract.