Mexican soldiers foiled the construction of a suspected drug tunnel underneath a house near the US border, arresting nine people.

Wired with lights, the tunnel was being built next to the Tijuana border crossing, south of San Diego, California, and near a Mexican air force installation as well as a regional federal police facility. The nine people arrested were reported to work in the construction sector.

Mexico drug tunnels
A policeman checks the entrance to a suspected drug tunnel under construction inside a wardrobe in Tijuana on 7 April 2015Reuters
Mexico drug tunnels
A makeshift cart with containers is seen at a suspected drug tunnel under construction in Tijuana on 7 April 2015Reuters

Over the past decade, dozens of illegal tunnels have been found across the roughly 2,000 mile (3,200km) US-Mexico border.

In May 2014, authorities discovered an incomplete tunnel near the US-Mexico border in Arizona. The tunnel, roughly 140ft long, was the fourth cross-border tunnel discovered in the area that year. As soon as one tunnel is shut down, the Mexican drug cartels start work on another.

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

In February 2014, authorities released photos of tunnels under one of the hideouts of Joaquín Guzman Loera, aka El Chapo. While not used to smuggle drugs between Mexico and the US, they did help one of Mexico's most notorious drug lords escape (if only briefly – he was caught soon afterwards).

Seven homes were connected by tunnels and to the city's sewer system. The house doors were reinforced with steel, which delayed entry by law enforcement, presumably allowing Guzman to flee.

Mexico drug tunnels
A Mexican marine lifts a bathtub leading to a tunnel at one of the houses of Joaquin "Chapo" Guzman in Culiacan, on 27 February 2014Daniel Becerril/Reuters
Mexico drug tunnels
The bottom of a removable bathtub is pictured from inside a tunnel leading to the city's drainage system at one of the houses of Joaquin "Chapo" Guzman in Culiacan, Mexico, on 25 February 2014Daniel Becerril/Reuters
Mexico drug tunnels
A journalist investigates a steel door leading from the city's drainage system into a tunnel underneath one of the houses of Joaquin "Chapo" Guzman in Culiacan, on 25 February 2014Daniel Becerril/Reuters

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. In 2014 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.

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

entrance
4 April 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
warehouse
4 April 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
tunnel
31 October 2013: A Homeland Security agent enters a tunnel used to smuggle drugs between the Otay Mesa in San Diego and TijuanaReuters
drugs
31 October 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
warehouse
31 October 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 a rail system for smuggling drugs between a San Diego industrial park and Tijuana, MexicoReuters
warehouse
12 July 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
vent
5 January 2013: A ventilated tunnel is seen in a cellar in Mexicali, MexicoReuters
tunnel
6 December 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
tunnel
12 July 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
meth
13 July 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
Mexico drug tunnels
Agents prepare to enter a tunnel found in a warehouse in Otay Mesa, California, leading to a small business building in Tijuana, on 30 November 2011. The tunnel was kitted out with an elevator, wood flooring and lightingSandy Huffaker/Getty Images
Mexico drug tunnels
An official investigates a sophisticated tunnel found in a warehouse in Otay Mesa, California, on 30 November 2011Sandy Huffaker/Getty Images
tunnel shoes
29 November 2011: Packages containing marijuana are seen at the Mexican entrance to a tunnel running between Tijuana and San DiegoReuters
tunnel
30 November 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
marijuana
30 November 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
police
29 November 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
tunnel
16 November 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
16 November 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
Mexico drug tunnels
A Mexican soldier patrols a 700m tunnel discovered at a warehouse in Tijuana, Baja California state, Mexico, leading to San Diego, on 26 November 2010Francisco Vega/AFP
tunnel entracen
3 November 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
marijuana
25 November 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
police warehouse
3 November 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 smugglers' tunnel the length of six football fields linking Southern California with MexicoReuters
Mexico drug tunnels
The view out of a drug tunnel found by agents in a warehouse near the US-Mexico border on 4 November 2010. Authorities confiscated over 30 tons of marijuana in the tunnel, which connected to a warehouse in TijuanaSandy Huffaker/Getty Images
Mexico drug tunnels
A homeland security agent crawls through a tunnel connecting a warehouse in Otay Mesa, California, to a warehouse in Tijuana, Mexico, on 4 November 2010Sandy Huffaker/Getty Images
virgin
4 December 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
Mexico drug tunnels
US immigration and customs enforcement officers stand guard over an entrance to a drug tunnel found in a warehouse in Otay Mesa, California, on 30 January 2006. The tunnel was 2,400ft long and furnished with lighting, ventilation and equipment to pump out ground waterSandy Huffaker/Getty Images
Mexico drug tunnels
A US police officer stands in a tunnel found along the Mexico/USA border at a warehouse in Otay Mesa, CaliforniaSandy Huffaker/Getty Images