National boundaries are nice, clean methods of distinguishing who owns what. If something is on one side of the border, it belongs to one country; if it is on the other side, it belongs to another. That seems simple enough that anyone can understand it. The problem arises when boundaries change, and everyone seems to forget about something as trivial and inconsequential as — let’s say — a whole town.

Río Rico is a town located along the Rio Grande River in the Mexican state of Tamaulipas. The Rio Grande river has been the agreed-upon boundary between the United States and Mexico since 1845. Despite being on the south side of the river, Río Rico was a territory of the United States for the first half of its history.

The Rio Grande river is known for its winding, meandering path. The area on which Río Rico was established existed within a particularly-twisty piece of the river, nearly encircling the territory. That all changed in 1906 when the Rio Grande Land and Irrigation Company diverted the river by eliminating the partial loop and making it continue, unimpeded, without the deviation. This had the effect of placing the 413 acres on and around Río Rico on the Mexican side of the river. This alienated section of U.S. property became known as the Horcon Tract.

There were more than a few issues surrounding the action of the Rio Grande Land and Irrigation Company. For one thing, it did this without prior authorization. Secondly, prior authorization would have been unlikely, anyway, since the move was prohibited under the terms of a treaty between the United States and Mexico.

The Rio Grande Land and Irrigation Company was fined and ordered to put up boundary markers, clearly showing the real estate that remained part of the United States. The latter part of this order did not get carried out, however. This led to increasing murkiness over the true ownership of the land.

During the 1920s and 1930s, Río Rico developed as a tourist spot for those who wished to escape the restrictions of Prohibition. Although the sale and consumption of alcoholic beverages were prohibited throughout the United States, this was not enforced in Río Rico. It was still regarded as a U.S. community, but its location south of the Rio Grande made enforcement of Prohibition fairly lax. During this time, Río Rico became a bit of a Las Vegas of its day, with free-flowing alcohol and gambling for anyone who wished to make the journey.

With the repeal of Prohibition, Río Rico’s celebrity status faded. The government of Mexico assumed custody of the community, but the United States had not formally ceded it. The arrangement was not intentional; it just wasn’t on anyone’s radar, and it just happened. It wasn’t until 1967, when James Hill, Jr., a geography professor at Arizona State University, rediscovered unusual circumstances 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 Horcon Tract to the Republic of Mexico.

This was not the end of the story, however. In 1972, Homero Cantú Treviño, a resident of Río Rico, filed suit to prevent the United States Immigration and Naturalization Service from deporting him from Texas to Mexico. He argued that he was an American citizen by virtue of having been born in Río Rico while it was still, technically, U.S. territory. The court initially ruled against him, but on appeal, the court held that the 1906 river diversion was unauthorized. Consequently, anyone born in Río Rico from 1906 to 1972 was entitled to United States citizenship. The ruling almost emptied the city of residents, since they were now able to move to other areas of the United States as full citizens.

Thus, the story of Río Rico is added to the list of ambiguous geographical oddities, such as the island that changes nationality every six months, the country that drifts through the ocean, and the military fort that was accidentally built in the wrong country. For such a small community, it has certainly built up a rich and interesting history.