The three, all from Surrey, British Columbia, were arrested last July, shortly after they finished the 360-foot tunnel just north of Lynden. It ran from the living room of a home on the U.S. side to a boarded-up Quonset hut on the Canadian side.

Prosecutors said Francis Devandra Raj, 31; Timothy Woo, 35; and Jonathan Valenzuela, 28, spent a year working on the tunnel, which cost an estimated $400,000.

Before they finished it, though, border guards noticed them bringing construction materials into the hut, and loads of dirt out. Investigators used the Patriot Act's provision for "sneak-and-peek" search warrants to examine the tunnel from the U.S. side and set up cameras to monitor it.

The investigators allowed three marijuana-running trips to take place in the tunnel in hopes of learning more about the suspects and whether they were involved in a wider drug ring. In each case the defendants were tailed as they left the tunnel.

"The tunnel would have posed even more serious risks if it went undetected," prosecutors wrote to U.S. District Judge John C. Coughenour in a sentencing memorandum. "Just as cars and planes have become vehicles for the transportation of a wide variety of contraband and people, the tunnel could easily have served other nefarious purposes had it not been closed so quickly."

All three men pleaded guilty to conspiracy to import marijuana.

The tunnel was filled in following the arrests.