- By UTV Correspondent Mark McFadden, who has lived close to the Irish border his whole life and been a reporter in Derry for 30 years.
Winston Churchill famously viewed Russia as a riddle, wrapped in a mystery, inside an enigma. If the grand old man - the epitome of British bulldog spirit - were alive today, he might well take a similar view of the border in Ireland.
The Irish border is the UK's only land frontier - a meandering line that stretches for more than 300 miles. But it's the biggest stumbling block on the road to a smooth and orderly exit from the European Union.
Together with ITV's Tonight team, I took a road trip along the border to find out why it has proved such a headache for Westminster.
I've lived close to the border all my life. I've been a journalist reporting on decades of violence and the moves towards peace. But let me make it plain that my personal history lends context, nothing more. It certainly does not confer expertise.
At times I too struggle to understand the subtle nuances that have made the Irish border such an incendiary topic. That's why I took to the road. Meeting the families who live, work and socialise on both sides of the border.
Every day these families cross the line without even noticing. They told me that more than 20 years of peace has transformed their lives. The days of military fortresses, bombs and bullets have gone. The border has become effectively invisible - and that has allowed communities to reach out, to live as one.
Driving along the border presents a bizarre challenge. If it wasn't for road signs changing from miles per hours to kilometres per hour, you might not even realise you'd left the UK and entered the Republic of Ireland. Some roads will change their national status several times within just a few miles. There are routes where one side of the road is in the UK and the other in the Republic. Simply overtaking another vehicle can mean crossing the border and then crossing back again.
How can the UK government and the European Union produce a workable arrangement that allows the border to remain open?
The EU and UK government agreed to ‘the backstop’. It's a mechanism which would keep the UK in a customs union and maintain an open border in Ireland. But the backstop has proved unpopular - even "toxic" - to those who want to sever all ties with Europe and draw up new trade deals.
However, if there's no harmony on customs arrangements, or food standards, or medicines, then it seems inevitable that some checks - perhaps even physical inspections - may be needed.
But the prospect of checkpoints on the border is awakening memories of Northern Ireland's dark past... a time when the border was a flashpoint.
I met those who had suffered from violence on the border. In 1990, Kathleen Gillespie's husband Patsy was forced to drive a van laden with explosives into an army checkpoint. He and five soldiers died. Kathleen says there must be no return to border posts.
Some have argued that technology could be the answer, with cameras and GPS tracking to monitor people, goods and traffic crossing the border.
But I met security expert Brian Gormally on the border. He thinks that even minor infrastructure could spark violence. He predicts cameras would need to be defended, and before long a simple camera would evolve - becoming a police outpost, then an army checkpoint and eventually a fortress.
There are concerns, too, from business and industry. At the moment goods flow freely and seamlessly across the border. But if checks and delays are introduced, trade could be damaged. Time, after all, is money.
I spoke with farmers and factory owners, even funeral directors. The dead cross the border in Ireland just about every week. In the border village of Belleek, undertaker Patsy McCauley told me he doesn't want to return to days when coffins were searched by soldiers and police.
Sportswear manufacturer Kieran Kennedy employs more than 700 workers from both sides of the border. His factory is in Strabane - two miles from the border. Most of his goods are bound for export to the EU and further afield. Already he is considering moving the entire operation into Donegal at a cost of 20 million Euro.
My final destination took in a muddy sports field in Newry where the constitutional question was front and centre. I watched a community rugby match with players from both sides of the border. In Ireland, rugby is played on an all-island basis. On the sidelines, spectators were talking openly about how Brexit has energised those who want to stay in the EU - and that would mean a united Ireland. Such a scenario may be a long way off, but Brexit has fired up a constitutional debate along the border.
My road trip was never going to find solutions to the myriad complexities of Brexit. After all, two years of painstaking negotiations - and three Brexit ministers - have so far failed to reconcile the opposing views.
But my journey did reveal just how communities on either side of the border have become intertwined and interdependent, and how so much of life operates on a cross-border basis. There are agreements on health care, education, electricity supplies, food and farming standards, transport, culture and art. Will all of that be challenged - or even put at risk - by Brexit?
The border may be distant to people in England, Scotland and Wales. But in Ireland, it's a problem that threatens to divide communities and families and businesses. The invisible line may be about to become all too visible, and all too real.