Border Walls

An estimated 11 million to 12 million undocumented immigrants live and work in the United States, roughly one in every 20 workers, according to a study by the Pew Hispanic Center based on government figures.

The nearly 2,000-mile (3,200-km) border with Mexico is the principal route of entry for illegal immigrants.

The U.S. Border Patrol arrested some 880,000 people crossing the border illegally in 2007, most of them from Mexico and Central America, down from 1.1 million a year earlier.

The U.S. government had built 284 miles of pedestrian and vehicular fence along the U.S.-Mexico border by the close of 2007, and aims to complete roughly 670 miles by the end of 2008.

The U.S. Border Patrol now has roughly 15,000 agents deployed on the Mexico and Canadian borders, and aims to have more than 18,300 agents by the end of 2008 -- more than double the number since President Bush took office in 2001.

The U.S. government signed off on an experimental stretch of hi-tech "virtual fence" last month, built by Boeing Co along a 28 mile section of the Arizona-Mexico border.

It consists of nine sensor towers equipped with cameras and ground radar, relaying a "common operating picture" to Border Patrol vehicles equipped with laptops.

The U.S. Customs and Border Protection agency has a growing fleet of Predator B unmanned surveillance drones.

Three were in operation at the start of the year on the Mexico border, a fourth Predator drone is due to be deployed to Grand Forks, North Dakota, on the Canadian border, in 2008.

The United States has 5,560 miles of borders with Canada, including 1,550 miles in Alaska.