U.S. Border Patrol agents have uncovered a sophisticated underground tunnel stretching more than 1,000 feet into the United States from Mexico as part of a drug-smuggling operation in Southern California.

An unfinished tunnel for “large-scale narcotics smuggling” from Tijuana to San Diego under the U.S.-Mexico border has been uncovered, federal officials reported. The “highly sophisticated” tunnel extended more than 1,000 feet into the United States, spanning 2,918 feet total, the U.S. Border Patrol said in a June 18 news release.