Authorities have discovered an incomplete tunnel near the US-Mexico border in Arizona. The tunnel, roughly 140 feet long, is the fourth cross-border tunnel discovered in the area this year. As soon as one tunnel is shut down, the Mexican drug cartels start work on another.

May 22, 2014: An underground tunnel near the US-Mexico border is filled with concrete in Arizona. The tunnel is the fourth one found this yearReuters
May 22, 2014: Workers fill an underground tunnel next to the US-Mexico border in Arizona with concrete. The incomplete tunnel is roughly 140 feet longReuters

The drug-smuggling tunnels are sometimes very sophisticated. In October 2013, authorities discovered a 'super tunnel' the length of six football fields, equipped with reinforced walls, lighting, a ventilation system and a rail system.

Cross-border tunnels can take many months and cost hundreds of thousands, if not millions of dollars, to construct. But the cost can be offset in just one quick shipment of marijuana, cocaine or methamphetamines. Earlier this year police seized 44 tons of marijuana in Tijuana, just across the border from San Diego, California. A record haul of 150 tons of marijuana was confiscated in Mexico in 2010.

IBTimesUK looks at some of the recent tunnels that have been found linking the US and Mexico.

April 4, 2014: Law enforcement officers inspect a drug tunnel found in a warehouse in Otay Mesa, San Diego. US federal agents uncovered two drug-smuggling tunnels underneath the US-Mexico border, both surfacing in San Diego-area warehouses and equipped with rail systems for moving contraband. The discovery led to the arrest of a 73-year-old woman accused of running one of the warehouses connected to a drug smuggling operationReuters
April 4, 2014: Mexican soldiers stand guard outside a building called "Mini Warehouses of the Border" in Tijuana, where authorities discovered a cross-border drug-smuggling tunnelReuters
October 31, 2013: A Homeland Security agent enters a tunnel used to smuggle drugs between the Otay Mesa in San Diego and TijuanaReuters
October 31, 2013: Seized drugs are shown in a sophisticated tunnel used to smuggle drugs between the US and Mexico. The tunnel linked an industrial park in Otay Mesa, California with Tijuana, Mexico and featured rail and ventilation systemsReuters
October 31, 2013: Law enforcement officials walk up to a boarded-up door of a warehouse in the Otay Mesa area of San Diego that leads to a drug smuggling tunnel. Authorities shut down a tunnel equipped with electricity, ventilation and rail system for smuggling drugs between a San Diego industrial park and Tijuana, MexicoReuters
July 12, 2012: A military truck is parked outside a recycling plant where soldiers found a tunnel under construction at the Otay Mesa industrial park in TijuanaReuters
January 5, 2013: A ventilated tunnel is seen in a cellar in Mexicali, MexicoReuters
December 6, 2012: Journalists climb down a ladder in a cross-border tunnel located underneath a water purifying plant in Tecate, Mexico. The tunnel, which was still under construction, was found due to an anonymous tip. Seven people who were found working on the tunnel when it was discovered were arrested by the authoritiesReuters
July 12, 2012: Soldiers inspect the entrance of a cross-border tunnel at a warehouse in Tijuana. The Mexican army located a tunnel measuring 383 yards (350 metres) running to California from Mexico and seized some 50 tons of marijuana. The tunnel was still under construction but had ventilation and lightingReuters
July 13, 2012: Thirty-nine pounds of methamphetamines that were smuggled through a cross-border tunnel are pictured in this photo taken by the DEA in San Luis, ArizonaReuters
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November 29, 2011: Packages containing marijuana are seen at the Mexican entrance to a tunnel running between Tijuana and San DiegoReuters
November 30, 2011: A soldier looks down into a tunnel running under the US border from Tijuana. The tunnel was equipped with a hydraulic entry, an elevator and electric rail carsReuters
November 30, 2011: A soldier stands guard next to packages containing marijuana found in a tunnel in Tijuana . Authorities seized more than 32 tons of marijuana. The tunnel was equipped with a hydraulic entry, an elevator and electric rail carsReuters
November 29, 2011: Soldiers stand guard outside a warehouse where a cross-border tunnel was found in Tijuana. The tunnel measured around 800 metres and ran from the Mexican border city of Tijuana to Otay Mesa industrial park south of San DiegoReuters
November 16, 2011: A journalist walks through a tunnel in Tijuana . Police discovered a tunnel running to California from Mexico, and seized 14 tons of marijuanaReuters
capt america
November 16, 2011: Packages containing marijuana with stickers showing Captain America, a Sprite can and a bottle of Bud Light are seen in Tijuana . Police discovered a "major cross-border drug tunnel" linking warehouses in an industrial park south of San Diego and the Mexican border city of TijuanaReuters
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November 3, 2010: A Mexican soldier flashes his torchlight into a tunnel in Tijuana. Agents recovered more than 25 tonnes of marijuana in seizures related to the investigation in both California and Mexico and arrested a US citizen and his Mexican wifeReuters
November 25, 2010: A soldier sits on a cart inside a "narco-tunnel" found in Tijuana and crossing into the US.Reuters
November 25, 2010: A cache of marijuana is seen after it was recovered from a tractor trailer truck found near a warehouse in San Diego. US border agents found a half-mile-long tunnel under the US-Mexico border and seized a significant amount of marijuana at the San Diego area warehouse where it endedReuters
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November 3, 2010: An agent from the US Immigration and Customs Enforcement (ICE) stands guard in front of an industrial building near the Mexican border in Otay Mesa, San Diego. Police found a sophisticated smuggler's tunnel the length of six football fields linking Southern California with MexicoReuters
December 4, 2007: An image of the Virgin of Guadalupe is seen in a tunnel, nearly a mile long, running into California from Tecate near the Pacific coast. Gunmen killed the police chief of Tecate, a Mexican city bordering California, by shooting him some 50 times in an apparent revenge attack after police found the drug-smuggling tunnel under the borderReuters