The US has invested millions of dollars in trying to keep out illegal immigrants
On the 20th anniversary of the fall of the Berlin Wall, BBC Mundo looks at barriers which are still standing - or have gone up since - around the world.
The border between Mexico and the United States is 3,200km (1,988 miles) long.
The US government has built a metal wall along a third of it, at an estimated cost so far of $2.5bn (£1.5bn), to prevent the arrival of illegal immigrants from Mexico and Central America.
The first barriers actually began to appear in 1991, but in 1994 the US officially decided to step up their surveillance and expanded the wall under Operation Guardian.
According to the Mexican National Commission of Human Rights, more than 5,600 illegal immigrants have died trying to cross the border in the subsequent years.
The majority died as a consequence of the high temperatures in the desert.
As well as the wall itself, there are also three metal fences in some places along the border, preventing any kind of contact at all. Its average height is 4-5m (13-16ft).
Construction of a "virtual wall" has also recently begun.
This comprises a series of technological devices such as infrared sensors, cameras, radar, watch towers and ground sensors.