BBC reporter Humphrey Hawksley (great name!) talks with US/Canada border enforcers.
In recent years, Canada has become a global hub of organised crime, run by mostly by Asian and motorcycle gangs. They have set up an intricate network to smuggle marijuana, counterfeit goods and guns into the US.
They have plenty of potential spots to cross.
The US-Canada border runs for more than 5,000 miles (8,000km) through some of the remotest areas in the world. It's often marked simply by a line cut in scrubland or a small obelisk.
But agents do what they can, both high-tech....
Even so, since 9/11, the US has been building what it describes as a "virtual fence" with an array of gadgetry that ranges from radiation detectors for nuclear weapons, to seismic sensors to catch people illegally sneaking across and number plate recognition so that immigration officers know pretty much who you are before you pull up at the booth.
Every agent of the US Border patrol is a trained tracker.
Agent Pinkerton and his team ride mustangs caught in the wild then broken in, because these horses are especially good at moving through rugged terrain.