Since 2005, there have been no checkpoints in place between Northern Ireland and the Republic of Ireland.

The border had once been dotted with British military checkpoints, and their removal was a landmark moment in the peace process that followed the violence of the Troubles.

Today, the only way you know you’ve crossed from one country to the next is a change in road signs from miles- to kilometres-per-hour.

But what happens to that 310-mile stretch of land after the UK leaves the EU is already proving one of the biggest challenges in the Brexit negotiations.

FactCheck takes a look at the options, and what it could mean for the peace process.

A non-existent land border is off the table

FactCheck spoke to Aoife O’Donoghue, Professor of Law at the University of Durham to understand the issue.

She explained that the no-border option is off the table: “that’s gone. If the UK had chosen to stay in the customs union and the single market, you could do that. But in deciding not to, that means there has to be some form of border.”

Academics at the Universities of Birmingham, Durham and Newcastle, including Professor O’Donoghue, explained why in a recent policy paper:

“Any approach which does not involved the UK remaining within the EU Single Market and the EU Customs Union will result in the imposition of some physical border controls because the legal guarantees of regulatory equivalence, mutual recognition, and non-barriers will require checks.”

This view was echoed by the EU’s chief negotiator, Michel Barnier, who told the Irish parliament that while he would work with all parties to avoid a hard border, “customs controls are part of EU border management. They protect the single market. They protect our food safety and our standards.”.

So it looks almost inevitable that some form of border checks will have to be put in place after Brexit if we leave the single market and the customs union.

A hard land border: almost inevitable if we leave the EU without a deal

Professor O’Donoghue explains that “the harder the Brexit, the harder the border.”

At what she calls the “very extreme” end of the spectrum is a hard border where the UK leaves with no deal and has to default to World Trade Organisation rules.

If the UK defaults to WTO rules (using copied-and-pasted versions of the EU’s tariffs in the short term), the EU would still have to maintain its side of the border. That would require check goods coming into Ireland from the UK.

That’s because the EU’s existence as a free trade area depends on its ability to demonstrate to the WTO that it can control its external borders properly.

A one-way land border: possible, but undesirable

The EU would need a physical border to check goods coming into the Republic of Ireland from Northern Ireland in the event of a no-deal Brexit.

Although in theory, the UK could decide not to impose checks on goods moving the other way (i.e. from the Republic into Northern Ireland). This could make a hard border slightly softer.

But there’s a catch: under WTO rules, unless you’re in a free trade bloc like the EU or NAFTA, you have to obey the “most favoured nation” rule.

That means if you lower trade tariffs for one trading partner, you have to lower tariffs to all your other partners. Professor O’Donoghue explains:

“If the UK chooses not impose any tariffs on goods coming across the [Irish] border… that would mean that the UK is giving the EU (because Ireland is the EU in this context) complete open access. So its most favoured nation tariff is zero. That means it would have to give a zero tariff access to every single country in the WTO.”

Now, the idea of the UK scrapping tariffs altogether isn’t entirely out of the question, according to some advocates for hard Brexit. Regular FactCheck readers might remember this proposal featured on the famous “Wetherspoon manifesto”, and was printed on half a million beermats across the pub chain’s 800-odd branches.

But doing so could have a devastating impact on UK businesses.

Back to our researchers at the Universities of Birmingham, Durham and Newcastle, who explain: “the impact upon the agri-food and farming sector is particularly revealing. Most agricultural products and livestock are subject to EU import tariffs of between 6% and 22%.”

“UK agri-food products would either have to compete with heavily subsided EU produce on the global market… or target sales within the UK to avoid import duties. It is likely that suppliers will use the cheapest available option which, due to CAP subsidies, may very well still be EU products”.

In other words, abolishing import tariffs could mean that UK producers are priced-out of UK and EU markets.

A “soft” land border is the DUP’s preferred option, but Dublin remains unconvinced

FactCheck also spoke to Professor John Garry of Queen’s University Belfast. He told us: “the DUP suggest that it is feasible to put into effect a border regime using technology – for example, with cameras and pre-registration of goods – and that that should be smooth and seamless.”

He suggested that “the DUP is frustrated that, in its view, the Dublin government hasn’t engaged as constructively as it could” with the idea that this kind of technology could make smooth border a reality.

But as Professor Garry explains, “the Dublin government says it’s far from being convinced that it’s technologically possible to have a north-south border that won’t cause difficulties. And what they mean by difficulties is both security-related and and economic.”

From Dublin’s perspective, even if a border between the Republic and Northern Ireland was made up of cameras logging vehicles in and out, this could still provide a target for those who wanted to abolish a border altogether.

A border in the Irish Sea: theoretically possible, but politically difficult

There has been speculation about whether the post-Brexit border could exist in the Irish Sea, rather than on land. That would mean the whole island of Ireland operating without an internal border.

Professor O’Donoghue explains: “the sea border would come about as a result of a special status being negotiated for Northern Ireland. That is possible, but politically that would be difficult from the perspective of the DUP.”

This week, the DUP MP for North Antrim, Ian Paisley, told the Irish Times that “we want to make sure the border is not moved to the Irish Sea, that is not acceptable ever and that would be a constitutional breach.”

The DUP leader Arlene Foster wrote to the leaders of 27 EU member states saying that she would categorically reject any arrangements that would give Northern Ireland a different status to the rest of the UK after Brexit.

And a sea border would raise a significant practical question: how would the UK prevent goods coming through the “backdoor” of Ireland?

Professor O’Donoghue explains: “Northern Ireland – in order to avoid a free for all for goods coming into the UK – would have to have a special status that would put it basically, maybe not in a customs union or a single market, but in a free trade area. That would be quite a substantial thing to negotiate.”

So a border in the Irish Sea is legally possible, but creates at least two political problems: convincing the Democratic Unionists on the one hand, and convincing the EU on the other.

What about the Good Friday agreement and the peace process?

Theresa May has described her government’s commitment to the Good Friday Agreement as “rock solid” and said “there is, and remains, strong support for the entirely peaceful future for Northern Ireland”.

The EU has also reaffirmed its commitment to the Good Friday Agreement, saying: “The Union has consistently supported the goal of peace and reconciliation enshrined in the Good Friday Agreement in all its parts.”

But, Professor O’Donoghue points out that: “The Good Friday Agreement envisages that there is equivalence between Northern Ireland and the Republic. It doesn’t mean that it has to be exactly the same (for example, there’s gay marriage in the Republic but not in Northern Ireland) but they’re supposed to be somewhat similar.”

She warns that if the UK government succeeds in getting rid of some elements of the Fundamental Charter of Human Rights after Brexit, Northern Irish law could be incompatible with the Good Friday Agreement.

One way round this problem could be to “keep [those elements of the Fundamental Charter of Human Rights] in Northern Ireland, and just get rid of them in Britain.”

But the risks of a hard border for the peace process extend beyond human rights law.

Professor O’Donoghue describes the situation in Norway and Sweden, where soldiers jointly patrol the border, and can pursue people who try to cross illegally. She says “that could be very sensitive, if you had the Gardaí [police] going into Northern Ireland…and of course vice versa with British troops in the Republic.”

Indeed, only this week, the Sinn Féin MP Chris Hazzard said that “if we see a situation where we are going to have border customs posts or any particular type of hardware, I would think that would be something people won’t want.”

He added: “and I go even further. An awful lot of the focus has been on maybe dissident republican organisations and threats, but it is wider than that – it goes right down to a feeling of civil disobedience.”