– It was "a very sociable summer," the Derry Girls recall. As easy as the breeze, they crossed the largely invisible border into the Republic of Ireland, to visit family, or share a pint, or swim in the sea during the unseasonably warm weather.

"So we all got to thinking how we take all this for granted — the freedom of it, the flow — and how it all could end," said Nicola Herron, 52, a local doctor who joined the group of like-minded women to pressure politicians to keep things just the way they are.

"It's scary, to be honest," said Elaine Doherty, 50, a psychologist and fellow activist in the campaign, which formally calls itself Derry Girls Against Borders. "Brexit is just months away — and there's not a single person who can tell you what will happen to us."

The 310-mile border that cuts across the island of Ireland has become perhaps the single greatest impediment in the divorce negotiations between Britain and the European Union.

"A real sticking point," as British Prime Minister Theresa May put it.

The challenges loom over how to continue to allow for the free movement of people and trade between the Republic of Ireland, which will remain in the European Union, and Northern Ireland, which will leave along with the rest of the United Kingdom.

And how to keep the border just as invisible, even as the United Kingdom and the European Union inexorably diverge — each free to establish their own immigration controls, customs tariffs and food safety rules.

And finally, how to do all this without upsetting the delicate peace in Northern Ireland that has relied on an open border.

People both north and south are quick to say there will be no returning to "the Troubles" — the vicious, intimate guerrilla war between pro-British Protestant unionists and Irish Catholic republicans that left more than 3,500 people dead.

Yet, sectarian lines remain deeply drawn in Northern Ireland. Many people in this border city — still known as Londonderry by Protestant residents and Derry by the 75 percent with Irish Catholic heritage — worry that a bungled Brexit could rekindle tensions and possibly lead to violence.

Today, driving along the Irish border, you might pass a farmer who has a barn in one country but grazes his sheep in the other. Almost 1 million people freely cross the squiggly line on the map each month. There are 200 official crossing points, and nobody knows how many dirt roads, foot trails and cow paths. The economies are tightly intertwined.

Border checkpoints, and all the militarized infrastructure of barracks, watchtowers, bunkers and blast walls, were removed from the island of Ireland in the aftermath of the 1998 Good Friday Agreement, a hard-won pact that ended 30 years of violence.

European Union membership made such evasion possible. E.U. policies of free movement and free trade allowed Northern Irish republicans to feel more connected to the Republic of Ireland, while unionists could continue to be an integral part of the United Kingdom. No one had to choose. Lines, grievances, identities could begin to soften.

But after Brexit?

Republicans worry that a defined border on the island would undercut their relationship with the rest of Ireland. Leaders of Sinn Fein, the republican political party, have warned that any Brexit border would hasten the day they seek an island-wide vote to unify.

British loyalists are livid about the E.U. proposal to place a customs border in the Irish Sea between Northern Ireland and Britain. May's governing partners, Northern Ireland's hard-line Democratic Unionist Party, oppose any kind of "special status" that would make them separate from the United Kingdom.

European Council President Donald Tusk has blamed the Brexit campaigners, "who are 100 percent responsible for bringing back the problem of the Irish border."

In the 2016 referendum, 56 percent of Northern Ireland voters cast their ballots to remain in the European Union. In Derry, it was 78 percent.

"Brexit has re-politicized everything," said Jennifer McKeever, president of the Londonderry Chamber of Commerce and owner of a shuttle bus service with a third of its staff and customers living across the border.

May and her European counterparts have promised there will never again be a hard border on the island of Ireland. But what's a real-world and politically feasible alternative? They haven't said, because they don't know.

May vows that her negotiators in the transition period after Brexit begins in March 2019 will craft an unprecedented free-trade accord with Europe that makes an Irish border unnecessary.

Time is running out on the Brexit negotiations. Frustrated by British delays, the Europeans have insisted on a "backstop," a legally binding insurance policy, to preserve an Ireland without borders in the event that a free-trade deal eludes them. In that case, Northern Ireland would remain a member of the E.U. customs union until the issue is resolved — a proposal that May has, until now, rejected.