U.S. and Mexican authorities uncovered an incomplete, solar-powered tunnel connecting the two countries in a remote area east of San Diego.
Border Patrol says the 600-foot long passage led into the small California town of Jacumba.
The tunnel had solar-powered lighting and a ventilation system.
It was also lined with a rail system that ran the entire length of the tunnel and had two sump pumps.
Mexican authorities discovered the tunnel back on September 19.
Officials have found many cross-border tunnels often incomplete.
They say the passages are often used to smuggle drugs from Mexico to the U.S.