An illegal, unfinished cross-border tunnel running into the U.S. was uncovered by authorities Monday, Customs and Border Protection announced.

The detection was made via a joint effort with Tucson Sector Border Patrol agents and Mexican law enforcement who were conducting a regular "bi-national tunnel sweep," the agency said in a news release Tuesday.

MEXICO THEME PARK STILL OFFERING ‘NIGHT WALK’ ILLEGAL BORDER CROSSING EXPERIENCE, AMID MIGRANT CRISIS

“The entry point was found submerged underwater along the international waterway channel constructed below both cities of Nogales,” CBP said. “The tunnel was adjacent to the Port of Nogales, DeConcini Crossing and came to an abrupt stop underneath a parking lot in Nogales, Arizona.”

Structural shoring and digging tools were found inside the roughly 50 foot long “illicit” passageway, suggesting to authorities that the construction was not yet finished, the news release said.

BORDER AGENTS SEIZE $1.7 MILLION IN METH, IN SECOND LARGEST DRUG BUST THIS WEEK

Of that length, 44 feet ran into the U.S., according to the agency.

“Agents will continue to monitor and inspect the incomplete tunnel until it is properly secured and remediated with concrete filler,” the news release said.