As a key part of the Brexit negotiations was about the border that separates Northern Ireland from the Republic of Ireland, TRT World's monthly series "Crossing the Line" is now investigating the effect of Brexit – whether it would revive the worst memories of a 30-year conflict, known as "the Troubles."

Today, the historic peace, known as the Good Friday Agreement signed in 1998, may be facing its greatest challenge with Brexit, which threatens to undermine the status quo of frictionless borders and free trade that has kept both Loyalist and Republican forces quiet for 20 years.

Series presenter Imran Garda travels across Northern Ireland meeting people from all sides of the Brexit debate to see how they are preparing for Brexit and what they fear or hope may come when the final deal (or no deal) is done.

In his investigation, Garda quickly discovers that old animosities continue to swirl close to the surface: Many communities remain segregated along ethnic lines, and paramilitaries claiming to fight for their respective communities unleash terror on innocents.

"To those guys on the other side of the fence, these are terrorists. To the people here, these are heroes. They're men who've fought for their country, for independence," says Garda.

The Northern Ireland episode of "Crossing the Line" premiers on TRT World this Saturday, Dec. 15 at 10:30 p.m.