Border Patrol agents discovered a 35-foot-long tunnel beneath the U.S.-Mexico border after it caved in and the asphalt roadway above it collapsed, officials said.

The tunnel ended in a patch of vacant land near the San Ysidro port of entry, said Lauren Mack, a spokeswoman for U.S. Immigration and Customs Enforcement. It was reinforced and was about 3 feet by 3 feet, and it appeared to have been used recently, she said.

It wasn't immediately clear when the tunnel was built or whether it might have been used for smuggling drugs or people.

Authorities sealed off the U.S. side with sandbags and metal after discovering it on Monday, Mack said.

The tunnel is across the border from an area that is either owned or leased by Mexican Customs.

Another tunnel was discovered Monday in Nogales, Ariz. Two men were arrested as they were hauling 300 pounds of marijuana from the 40-foot-long tunnel to a car, authorities said.

More than a dozen tunnels have been found along the southwestern border in recent years. In 2003, Border Patrol agents found a tunnel that originated in a private house in Mexico and ended in a parking lot on the U.S. side. A year later, U.S. authorities found evidence that someone was trying to rebuild the passageway.

"We do believe it's a trend where smugglers are attempting to go underground after 9/11 as a result of the heightened security," Mack said.