Two tunnels have been discovered under the US-Mexico border this week, American immigration agents say.
Hundreds of thousands are arrested along the border each year
One was discovered in Arizona when border patrol agents spotted two men taking marijuana out of it.
The other one was found in California after it caved in. Officials said there was evidence it had been used recently.
The discoveries come amid diplomatic tension between Mexico and Washington over a proposal in the US Congress to build more border security fencing.
The Arizona tunnel was discovered after officers, acting on a tip-off, caught two men with a 135-kilo (300lb) haul of marijuana.
Officials said the 12-metre (40-foot) tunnel ended in the driveway of a home in the city of Nogales.
The tunnel discovered near the Californian city of San Diego was about 10m (35ft) long and 1m (3ft) wide, officials said. It ended near the San Ysidro port of entry.
It was not clear whether the tunnel had been used for smuggling drugs or people, but rubbish found there suggested it had been in use recently, a spokeswoman for US Immigration and Customs Enforcement said.
Both tunnels have been sealed.
Hundreds of thousands of illegal immigrants, mostly Mexican, are arrested along the US south-west border each year.
The Senate is due to debate in February the proposed introduction of tougher border controls, including building more fences along a third of the border with Mexico.
The House of Representatives has already passed the bill, which has been condemned by Mexico.
The two countries share a 3,200-km (2,000-mile) border.