An incomplete tunnel found stretching from Mexico to Arizona appears to be "the most sophisticated tunnel in U.S. history," authorities said.

The tunnel intended for smuggling ran from a neighborhood in San Luis Rio Colorado, Mexico, to San Luis, Ariz., where it stopped short of reaching the surface. It had a ventilation system, water lines, electrical wiring, a rail system and extensive reinforcement, federal officials say.

"What makes this one unique is that the terrain in Yuma is very hard. … The sand is very loose, and most of them end up caving," said Angel Ortiz, assistant special agent in charge for Homeland Security Investigations in Yuma.

"This appears to be the most sophisticated tunnel in U.S. history, and certainly the most sophisticated I've seen in my career," said Carl Landrum, acting chief patrol agent with the Border Patrol's Yuma Sector.

ASSOCIATED PRESS