U.S. Customs and Border Protection (CBP) officers discovered a smuggling tunnel under construction.

Tucson Sector Border Patrol agents discovered the tunnel after a construction worker, who was clearing old border fencing, noticed the opening and notified agents, the CBP reported.

Agents from the Brian A. Terry Station investigated the tunnel, which was 15 feet-deep and extended four feet into the United States. The structure had no lighting or ventilation and was most likely still under construction upon discovery. The tunnel will be filled with concrete.

The discovery of the tunnel comes just days after Tucson Sector Border Patrol agents found another smuggling tunnel.

The tunnel discovered on Wednesday, which extended into the U.S. by 60 yards, was completed but collapsed at some point.

Ryan Saavedra is a contributor for Breitbart Texas and can be found on Twitter at @RealSaavedra.