The Irish border starts just in front of these signs in County Loudonderry. (Photo: Jonathan Billinger/CC BY-SA 2.0)

The border of Ireland and Northern Ireland has, for years, been almost totally porous. There are no customs agents. There are no passport checks. In many places, you wouldn’t even notice you were crossing between the two neighboring states. Essentially, there are no rules.

But that might have to soon change, after Thursday’s historic vote in the United Kingdom to leave the European Union. That’s because Ireland will remain a part of the E.U., while Northern Ireland, a constituent part of the U.K., will not.

What could this mean? A lot of things, though any real changes to how the border is protected likely won’t happen for years, if at all, in part because it will take at least two years for Britain to formally exit the E.U. Even then, some say a solution keeping the border mostly open could be negotiated.

But even the smallest changes could represent a huge disruption to the economies in both states, since up to 20,000 commute between the border daily, in addition to massive free trade of goods. And we haven’t even mentioned the animals.

“There is constant daily movement of live animals for fattening, and dairy products, to name just two elements of the trading relationship,” Phil Hogan, the E.U.’s agriculture commisioner said, according to the Irish Times.

Customs checks were removed in 1993 following the formal creation of the Eurozone, but even before that, and even during the height of The Troubles, you generally didn’t need a passport to cross between the two states.

And in the wake of the Brexit vote, many—typically those who voted to leave the E.U.—have argued that this situation can remain the same, the two states’ historic economic ties too strong and too important to sever. But many who voted to remain in the E.U. have argued the opposite: border patrols by definition will have to be stepped up, since the E.U. likely wouldn’t allow one of its members to have a different immigration policy than the rest of the bloc.

Still, the border represents a real, and symbolic, divide on the island, between the predominantly Catholic Ireland and Northern Ireland, which is sharply split between Catholics and Protestants. Some worried that any heightened patrols at the border might revive old rivalries, officially settled by the Good Friday Agreement in 1998.

But others said that Brexit in the end might have a completely different effect: unification. Northern Ireland largely voted to stay in the E.U. and, as the Irish Times points out, reaps a huge number of economic benefits from its membership, especially for farmers.

And, on Friday, the leader of Sinn Fein, a major Irish political party that has supported unification, said he would call for a new referendum to decide the island’s future.

The framework for unification is certainly there as each state just needs to approve unifying with a simple majority. A unified island, of course, wouldn’t need to worry about the border at all.

The headline of this article has been corrected. Great Britain also shares a border in Gibraltar with the E.U.