Police found a partially constructed tunnel near the U.S. border in Mexicali, the capital of the northern Mexican state of Baja California, officials said.
The tunnel was presumably being built by drug traffickers, Mexicali police chief Marco Antonio Carrillo said in a Twitter posting that included photographs.
The "narcotunnel" was found in a joint operation with the army, the police chief said.
The tunnel is 10 meters (nearly 33 feet) deep, 30 meters (98 feet) long and 50 centimeters (1.6 feet) wide, Carrillo said.
The illegal tunnel has cement walls, lighting and a ventilation system, Carrillo said on the social-networking site.
The tunnel was built inside a warehouse in downtown Mexicali and was found thanks to an anonymous tip, the police chief said.
The people building the tunnel were unable to complete it, Carrillo said.
Unconfirmed media reports said a suspect was arrested at the site.
Since the mid-1990s, authorities have discovered many clandestine tunnels along the border.
People traffickers and drug cartels often use clandestine tunnels to cross the vast U.S.-Mexican border, which runs 3,200 kilometers (1,988 miles). EFE