The US-Mexico border is known for two things: violence and smuggling; Americans talk about the border as a line that protects them from crime, drugs, and other bad things they see as Mexican problems.
David takes a road trip along the borderlands to explore the past, present, and future of US-Mexico border enforcement.
Today the UK-Ireland border is peaceful and open, but the conflict, rooted in the legacy of British colonialism, is hardly resolved.
For the last 22 years, Northern Ireland's fragile peace has been based on the fact that the Irish border is essentially open, but Brexit has thrown that arrangement into total uncertainty.
The Kenya-Tanzania border straddles some of the world's best known nature reserves and safari destinations. There, environmental and commercial concerns come together.
The land border between Kenya and Tanzania, combined with the Swahili Coast and its sea border on the Indian Ocean, is a crucial node in multiple international smuggling networks.
The long and grinding collapse of Venezuela's economy has pushed more than five million Venezuelans to leave the country in search of work, medical care, and other basic necessities.
The border between Colombia and Venezuela is officially closed - the international bridges that officially connect the two countries stand empty.