The Sabal Palm Sanctuary is a nature reserve southeast of Brownsville, Tex. It’s home to thousands of birds, which you can watch eat on a streaming webcam. The sanctuary was created on part of a plantation that once ran along the Rio Grande; the plantation’s mansion can still be visited by tourists.

Oh, and, according to Interior Secretary Ryan Zinke, it now belongs to Mexico.

Well, sort of.

While addressing the legislative conference of the Public Lands Council in Washington on Tuesday, the subject of President Trump’s proposed border wall came up.

“The border is complicated, as far as building a physical wall,” Zinke said, according to E&E News. “The Rio Grande, what side of the river are you going to put the wall? We’re not going to put it on our side and cede the river to Mexico. And we’re probably not going to put it in the middle of the river.”

Actually, you probably could build the wall in the middle of the river if you wanted to. For most of its length along the Texas border, it’s fairly shallow. (Here, see for yourself.) But, broadly speaking, there are obvious advantages to building the wall on land and, even more ideally, land that won’t constantly end up being submerged by the river when it runs higher than normal.

Zinke’s suggestion? We build it on the Mexican side of the river so we don’t “cede” that territory to our southern neighbors. There are a few problems with that idea, but one is more immediate than others: Under Zinke’s formulation, we’ve already “ceded” a lot of land to Mexico.

Here’s part of the border between Mexico and Texas near where the Rio Grande enters the Gulf of Mexico. Areas in white are in the United States; the brown line is the existing wall, miles and miles of which already exist. Everything south of the brown line that is colored white is land that has been “ceded” — including, in this case, the Sabal Palm Sanctuary.

(Wall data by Michael Corey, Reveal News. Thanks to Mapzen for help in creating it.)

There is a lot of territory that falls into that category, as you can see if you zoom out on the map above or as seen below. Everything between that orange line and that black line: ceded.

There have been any number of articles exploring this admittedly weird part of the United States. In August, a writer for the Rio Grande Guardian described swapping houses with a neighbor to get a sense of what life on the other side of the wall was like. In 2011, Liz Goodwin of Yahoo News covered Texans who lived on the south side of the wall, including some in Brownsville. “Technically, we’re in the United States,” one homeowner told Goodwin — though he noted that U.S. Border Patrol seemed to focus on defending the wall, not the actual border.

The number of people living in that gray area is hard to establish. Census tract data doesn’t break down that finely, and, for the most part, the area between the wall and the border is uninhabited because it’s a flood plain, as mentioned above.

To build the wall, the federal government would need to exercise eminent domain over a lot of territory owned by Americans. While Texas’s attorney general is perfectly comfortable with allowing the government to seize private land for its construction, some landowners are likely to disagree. (Particularly given that the border region is generally pretty Democratic.) Building the wall in Mexico would certainly avoid irritating those landowners.

It would, however, irritate Mexico.

Trump’s long-promised pledge to have the Mexican government pay for the wall was never realistic, and, as president, he has offered no plan to make that happen. It seems perhaps more unlikely that he will persuade a foreign nation to allow the United States to construct on its property, particularly given that the point of the wall is, among other things, to block the movement of people from that nation. Trump could theoretically get Mexico to “pay” for the wall by cutting off aid and funding to the country. But there’s essentially no way he could get Mexico to give up its territory.

Zinke’s overall point is correct: Building the wall involves balancing a lot of concerns and problems. How do you build in the mountains? Where do you run through environmentally sensitive areas? And how do you deal with the river? In the past, the river has been dealt with by isolating some small group of Americans on the other side of the barrier. In the future, it’s almost certain that the same compromise will be reached.

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