for years.

Trade's carried out freely with other member countries and people coming through only need

to show their EU passport.

But in June 2016, the UK voted to leave the EU so that it could reassert control on its

own borders - and decide who and what it wanted to let through.

Imagine these boundaries turning into hard borders.

The impact of that on these maritime borders is complicated in terms of trade,

but it could have serious implications for the people living along the UK's only

overland border — here.

This border, between Northern Ireland and the Republic of Ireland, is one of the reasons

why Brexit negotiations continue to reach a deadlock.

That's because this isn't just a boundary between two countries...

It's also a compromise.

A symbol of identity.

A solution to a troubled history.

And it's been keeping the peace in Northern Ireland for 20 years.

Hardening this border could put one of Europe's greatest success stories in jeopardy.

This border was first drawn in 1920 by the

British, who had ruled over the island for centuries.

The Irish had rebelled several times, but

not everyone wanted the British to leave.

So, eventually the UK divided the island into two states based on its population.

Most people in this part were historically Catholic, and identified as Irish, and wanted

independence.

They were known as Nationalists.

But in the North, many people were Protestant, identified more closely as British

and wanted to stay in the UK.

They were called Unionists.

After the partition, this part remained in the UK as Northern Ireland.

We made that decision as a people quite freely, and for very definite reasons.

Reasons that are historical, reasons that are cultural, and reasons that are economic.

The south continued to move away from the UK until it gained complete independence

and became a new country -- the Republic of Ireland.

At first, this 499 kilometer border was pretty porous.

But the UK and Ireland continued to be hostile.

Over time, customs checks were set up at the border crossings and the two countries descended into

a trade war - tariffs were placed on agricultural produce and goods like steel and coal.

By the late 1960s, things turned violent.

Violence like this hit Northern Ireland after years of simmering bitterness between the Catholic minority

and the ruling Protestant regime.

In Northern Ireland, fierce conflict broke out between extremist groups.

Nationalist paramilitaries, like the Irish Republican Army, believed that Northern Ireland

was rightfully part of Ireland and that the British were oppressors of Northern Ireland's

Nationalist population.

Unionist paramilitaries fought back; defending their place in the UK.

Both groups blew up buildings, set off car bombs, and engaged in bloody street fighting.

The UK deployed thousands of troops to Northern Ireland during this time; and became a common

target of Nationalist paramilitary attacks.

Especially at the border, which for Nationalists was the ultimate symbol of British occupation.

"Welsh fuseliers who patrol this stretch of the border

described in court as the main battle line between the IRA and the army,

have suffered repeated attacks."

As violence surged, the UK military tried to secure the border with walls, towers, heavy

guns, and patrols.

They tightly controlled the 20 official crossings

and screened people and vehicles passing through.

The conflict over Northern Ireland turned this into a hard border.

The violence lasted for more than 30 years, killed over 3,600 people and came to be known

as The Troubles.

It came to end in 1998, when Nationalist and Unionist Party leaders came together for

a historic peace deal.

They reached a compromise: Northern Ireland would remain in the UK but people would be

eligible for both Irish and UK citizenships.

And in the future, Northern Ireland could vote to join Ireland.

This deal came to be known as the Good Friday Agreement.

It allowed Nationalists in Northern Ireland to be part of the Republic of Ireland while

the Unionists remained part of the UK.

Which meant this hard border wasn't needed anymore.

So, the British military left.

The watchtowers came down.

And more roads opened.

There are now around 270 official crossings - most of which are completely invisible.

And they're all part of a border that stands as a symbol of the compromise that ended decades

of conflict.

"The British people have voted to leave the European Union."

"Reignited a fierce debate over Northern Ireland's future."

"Because both are members of the European Union.

But when Britain pulls out of the EU,"

"it's now an outer-EU border and the question is,

do we put up barbed wire again? Soldiers? There'll be a custom borders at the very least."

In June 2016, the UK voted to leave the EU, even though Northern Ireland was overwhelmingly

in favor of remaining.

The UK's argument in favor of Brexit was to control its own national borders — but

there was little mention of its Irish border at the time.

That changed when the UK and EU started negotiations -- the status of the Irish border became one

of the first three things to figure out.

Now, more than a year later, it's still unresolved.

But there are a few options: The UK could reimpose a hard border by bringing

back the police and the walls.

But that would isolate the population of Nationalists in Northern Ireland.

Alternatively, they could put the border here, leaving Northern Ireland in the EU Customs Union,

but separating it from the UK mainland.

But this would betray the Unionists.

See, either way, both these options risk violating the Good Friday Agreement.

A third option is for the UK to stay in the EU Customs Union meaning it wouldn't need a

customs border, but that's unacceptable for Brexiters in the UK government, who specifically

want control over their own borders.

The UK needs to put a border somewhere but just can't decide where.

“On relation to the border between Northern Ireland and Ireland,

we will not return to a hard border between Northern Ireland

and Ireland”.

“But the suggestion that there should be a border down the Irish Sea,

separating Northern Ireland from the rest of the

United Kingdom is completely unacceptable."

"We are not going to be in a customs union, we're not going to

be in the Customs Union, because if we were, that would prevent us

from being able to follow an independent trade policy.”

Now, there's a fourth option that would be in line with the Good Friday Agreement — it's

the idea of reunification.

In the past when both Ireland and the UK were in the EU and the borders were open; there

was little incentive for Northern Ireland to vote to reunite with the Republic of Ireland.

But if the UK went with the option of hard borders, Northern Ireland would be isolated

and the only way to rejoin the EU would be through reunification.

Typically, this would be an overwhelming victory for the Nationalists and a loss for the Unionists.

But Brexit seems to have changed some opinions.

A recent poll found that 28% of the respondents who supported Northern Ireland's place in

the UK would now vote to join the Republic of Ireland.

While not a perfect solution, it would give Northern Ireland a voice about its own place

in Europe; a voice that's barely been heard so far.