The US border may be unmarked in many places but it is under constant surveillance. Since its inception in 1924, the U.S border patrol has been tasked with preventing illegal crossings.
During World War II, Border Patrol was moved under the umbrella of the Department of Justice. And in 1952 for the first time, a legal entrance were subject to arrest. You will see the technology vehicles and methods that the U.S Customs and Border Protection use in order to do their job from helicopters to robots and snowmobiles agents have the latest technology to keep our nation secure.