Photos of secret underground cross-border smuggling routes between Mexico and US

More than a ton of cocaine and seven tons of marijuana have been seized after authorities discovered a tunnel lasting more than half a mile under the border between the US and Mexico. The tunnel ran for about 800 metres from an elevator fitted inside a closet in a house in Tijuana to a fenced-off area in the grounds of a business selling wooden pallets in San Diego. The tunnel was only about three-feet wide, equipped with a rail system, lighting and ventilation.

The discovery demonstrates the enduring appeal of tunnels to smugglers, despite the significant time and money required to build one. This is the 13th sophisticated secret passage found along California's border with Mexico since 2006, including three on the same short street in San Diego.

Dozens have been found along the US-Mexico border in recent years, mostly in California and Arizona. Many are found incomplete. The San Diego-Tijuana region is popular because its clay-like soil is relatively easy to dig into with shovels and pneumatic tools, and both sides of the border have warehouses that provide cover for trucks and heavy equipment.

In April 2015, 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.

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.

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.

Guzman wasn't in jail long – he escaped via a 50cm by 50cm entrance hole in the bottom of the shower area. There was a motorcycle modified to run on rails inside the tunnel, waiting for his escape. The tunnel led to an abandoned warehouse in Almoloya de Juarez, about a mile from the high-security prison.

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, ventilation 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.