The United Kingdom may be an island nation, but it has one, huge open border with the European Union. And closing it could cause very serious problems.
The Republic of Ireland, which is an EU member state, and Northern Ireland, which is part of the U.K., haven't been united for almost 100 years. In the past two decades though, the two have become intertwined in trade and culture.
The U.K.'s exit from the EU could change this dramatically. New border controls threaten to undermine two decades of peace between former sectarian rivals, and dampen economies in both Irish regions.
"There is fear in the north and in the south of Ireland that this is a step toward opening up an old wound that was healing wonderfully," said Peter Moloney, a visiting professor at Boston College, who studies the evolution of modern international governance.
The current border runs more than 300 miles east to west, almost invisible between farms and rolling hills.
Lines were stricter in the 1970s, as violence between Protestants and Catholics known as "Troubles" escalated. But even then, it was never an immigration border, Moloney said. It dealt primarily with custom and tariffs until 1973. The type of border that emerges post-Brexit will make a notable difference, he said.
"The danger is if you bring back checkpoints, if you bring back military towers that have been diffused for 20 years, you're bringing back nightmares from the past that people thought they left behind," Moloney said.
A main pillar of the "leave" campaign in the U.K. was claims that it is necessary to control immigrant flows into the country. But that would require the very type of borders that many Irish fear. The line between Ireland and Northern Ireland will now be the only land border between the United Kingdom and another EU country. If campaign promises are kept, it's about to become much less porous.
Martina Anderson, a member of the European Parliament for the Sinn Fein party, is still trying to prevent a Brexit despite the leave vote, and sees it as "live issue." She pointed out economic challenges in keeping the border as is.
"Europe is not going to allow the free movement of goods and people into its territory without there being some sort of checks and balances," Anderson said. "There's not going to be this back door wide open."
"There's 23,000 people cross the border every day to work or to study," she said. "Whatever border is erected would be problematic."Many of those students and workers looking to maintain dual access applied for passports in days following the Brexit vote. In London, the embassy received more than 4,000 passport requests, compared to the 200 a day it usually gets, Reuters reported this week. Anderson said Belfast is also at capacity with requests.
"The increased interest clearly points to a sense of concern among some U.K. passport holders that the rights they enjoy as EU citizens are about to abruptly end," Minister for Foreign Affairs and Trade Charlie Flanagan said in a statement. He urged people to slow down, saying there's "absolutely no urgency for U.K. citizens, who may also be Irish citizens, in now applying for Irish passports."
Those applying for passports could benefit from access to a potential new financial hub: Dublin.
In the short term, Brexit may prop up the country's capital. Dublin is the last English-speaking capital in the EU, and London-based companies may be looking for a new place to call on the rest of continental Europe.
"There's no uncertainty, about Ireland's position in the EU in the long run, and I think that will help the financial center," said David Kelly, chief global strategist at J.P. Morgan.
Long-term however, things aren't looking so cheery. The United Kingdom is by far Ireland's most important trading partner, said Kelly, who is originally from Dublin but lives in New York. Fitch Ratings warned about the country's exposure this week, saying Ireland was among the countries most vulnerable to EU shocks because of changing trade relations with their neighbors.
Ireland will have to work with other European partners to maintain any sort of free trade between itself and the U.K., especially when it comes to Northern Ireland.
"The last thing they'll want to see is tariffs go up all over the place," Kelly said. "I think the government will do as much as they can to try and get a not-too-onerous deal for Britain."
Northern Ireland, a part of the United Kingdom, is at risk from a move that the majority of people did not vote for. Fifty-six percent of citizens in Northern Ireland voted in the referendum to stay in the EU.
The region will lose free trade with its southern neighbor and funding from the rest of the EU.
"The North is going to be seen as the ugly duckling on the dance floor," Anderson said, adding that a key reason investors were drawn to Belfast in the past two decades was access to the 500 million customers in Europe.
"It's as if nobody wants to dance with us, nobody wants to partner with us because there is that uncertainty with the direction of travel that the British government wants to take."
Farmers in Northern Ireland are also set to lose EU subsidies after Brexit. The Common Agricultural Policy, or CAP, has supported farmers in Scotland and Ireland since 1973.
The United Kingdom is now solely responsible for compensating Scotland and Northern Ireland, and replacing money they would have received from the EU. The 350 million euros per week ($390 million) the leave campaign said they will save by not paying taxes to the EU may be stretched too thin, Moloney said.
"They've earmarked that money for so many different projects, it's hard to believe there's going to be enough money left for Northern Ireland or for Scotland," Moloney said.
It's not only farmers who could reel from lack of funding from the European Union.
Seamas Heaney, who directs a health center in the small border town of Derry, also benefited from EU funding. His Healthy Living Center at the Old Library Trust supports disadvantaged people over 60 suffering from dementia and type II Diabetes.
Heaney's team recently finished drafting a research plan that relied on financial support from the EU.
"A lot of the plans we put forward could all come to not," Heaney said, adding that post-Brexit they have to come up with alternative funding from the United Kingdom, which has already gone through austerity measures.
"We're already on a shoestring budget of funding," he said. "It is difficult to know if we can sustain our services in an increasingly hostile economic environment.
"It's a gloomy picture, but it's like anything else in the world, you just have to get on with it," Heaney said.