Perhaps the most discernible aspect of the border between the Republic of Ireland and Northern Ireland is just how indiscernible it is. With few signposts marking the end of one jurisdiction and the beginning of another, travelers going between the two rely on subtle clues indicating they’ve crossed from one country into another—the changing color of road signs, speed limits switching from kilometers (the Republic) to miles (Northern Ireland), or the more obvious shift between sightings of the Irish tricolor and the Union Jack.  

But this seemingly invisible border may not stay that way for long. With the United Kingdom’s vote last summer to leave the European Union, both parties now face the challenge of determining what the border between Northern Ireland (a part of the U.K.) and the Republic of Ireland (a member of the EU) should look like. While British and European leaders have voiced their opposition to implementing a so-called hard border, the extent to which it will be controlled is still up for negotiation. Alongside the issues of citizens’ rights and the U.K.’s financial obligations to the EU, the border issue has emerged as one of the most contentious in Brexit talks thus far. EU leaders have threatened not to move forward on other topics, such as working out a trade deal between the U.K. and the EU, before all three issues are addressed.

The Irish border wasn’t always so invisible. Between the 1960s and the 1990s, during a period of cross-border sectarian fighting known as “The Troubles,” the heavily-fortified border between the Republic of Ireland and Northern Ireland was far more distinct, outlined by military checkpoints, watchtowers, and customs posts. More than three decades of fighting between the Unionists (predominantly Protestant) advocating for Northern Ireland to remain part of the U.K. and the Republicans (predominantly Catholic) advocating for the North to join the Republic of Ireland culminated in the Good Friday Agreement. This 1998 accord brought an end to the conflict and established a system in which power between the Unionists and the Republicans would be shared. As the fighting between the North and the Republic receded, so did the militarized frontier that once divided them.

“Now if you travel between the Republic of Ireland and Northern Ireland, it’s like driving from New York to Connecticut—you just keep going,” Senator George Mitchell, the primary architect of the Good Friday Agreement, told me. “In fact, it’s hard to even know it now.”

Though the implementation of a hard border certainly wouldn’t resemble the wartime frontier that defined the Troubles, Mitchell said certain aspects, such as the reimposition of border controls, could bring echoes of past division and conflict that might threaten the cooperation the Good Friday Agreement helped achieve. “Commerce has increased, economies have become increasingly integrated … demonization has declined as people move freely across the borders and get to realize they have more in common than that which divides them,” Mitchell said, adding: “A reinstitution of a hard border would set all that back.”

The challenge facing British and European negotiators is how to reconcile the U.K.’s decision to leave the EU with the bloc’s customs union and single market, without implementing a hard border. This is particularly important when it comes to trade between the EU and the U.K. and, more specifically, between Ireland and Northern Ireland. According to a December 2016 report by the U.K. House of Lords, approximately 60 billion euros are traded between the U.K. and Ireland each year, and as many as 30,000 people are estimated to commute across the border between Ireland and Northern Ireland each weekday for work. Between members of the EU, such trade is not subject to tariffs, nor are the individuals crossing the border subject to passport checks.

That all changes, however, once the U.K. formally leaves the EU. “Under EU law, it would end the free movement of people, goods, labor, services, and capital,” Judy Dempsey, a nonresident senior fellow at Carnegie Europe, told me. “Essentially, Northern Ireland would be cut off from Ireland, and Irish trade would be hugely damaged because Ireland in turn would not be able to sell its goods freely to Great Britain, and vice versa, because EU law would have to apply.”

While the U.K. could decide not to impose customs controls on the Northern Ireland side of the border, European law requires the implementation of some sort of customs control on the Republic’s side, and EU leaders have stressed that some customs controls would have to be enforced. “The UK’s departure from the EU will have consequences,” Michel Barnier, the EU’s chief Brexit negotiator, told Ireland’s parliament in May. “Customs controls are part of EU border management. They protect the single market. They protect our food safety and our standards.”

One industry that stands to be among the hardest hit by increased trade barriers is Northern Ireland’s dairy farms, which rely on cross-border production with Ireland to produce their products. “We are very, very dependent on what we call an all-island value chain,” Mike Johnston, the Northern Ireland director for Dairy U.K., told the House of Commons’ Northern Ireland affairs committee in January. “If we have any interruption in the current practices, it is going to affect the longer-term viability of the industry.”

Dairy isn’t the only industry that’s worried. Diageo, the multinational company that produces Guinness and Bailey’s Irish Cream, warned that its prices could rise if trade restrictions are imposed, noting that some of its products cross the border as many as three times before being exported in bottles. “It’ll be a disaster for everyone if that border once again becomes a closed border,” Mitchell said. “Beyond the border itself—the physical border—is the full integration of the Irish and U.K. economies in a way that I think requires a special provision in any Brexit agreement between the U.K. and the EU.”

A special provision isn’t out of the realm of possibility. U.K. and EU leaders have both expressed their willingness to maintain the Common Travel Area, a 1922 agreement that allows British and Irish citizens to travel freely between the two countries (though this agreement would not apply to non-Irish and non-British EU citizens who currently commute between the two). But it’s unclear what this special status would look like, or whether Northern Irish leaders would support it. Sinn Féin, Northern Ireland’s second-largest party and one that advocated for the U.K. to remain in the EU, supports establishing a special status. But the Democratic Unionist Party (DUP), Northern Ireland’s largest political party and one that backs Prime Minister Theresa May’s government, has warned that it will oppose any status that would allow the North to remain in the EU.  

But if it comes down to maintaining the peace, Dempsey said a special status could be achieved. “If there’s a political will, it’s possible,” she said. “Germany is very important in this role … and Merkel understands completely the idea of a special status and she would probably make an exception for Northern Ireland because what’s at issue is stability—it’s stability on Europe’s western frontier.”