For a couple of years, it looked like America's longstanding problem of illegal immigration from Mexico was finally being solved. After the September 11th terror attacks, the illegal flow of people across America's southern border slowed. Arrests fell. The decline may have resulted from new initiatives by the US Border Patrol. It may also have resulted from fewer Mexicans trying to cross the border, given expectations of increased enforcement post-September 11.

Whatever the reason for the decline, it didn't last. In the past 12 months, border patrol agents have apprehended more than a million illegal migrants from Mexico. All this week, we'll be bringing you stories from the border. We'll hear from border patrol agents, migrants, aid workers... and American citizens worried about terrorists trying to sneak into the United States from the South.

Today, The World's Patrick Cox reports that in the past decade, the border has been fortified with fences and high-tech defenses along certain stretches... but overall, it remains as porous as ever.