Border agents have discovered what could be the longest cross-border tunnel in history, seizing seven tons of marijuana and a ton of cocaine that were crammed inside.

The 2,600ft tunnel extended 900ft from a house in Tijuana, Mexico, and across the border for a further 1,500ft, where it surfaced in a fenced-off industrial area in San Diego.

The tunnel, discovered just in time for 4/20, the day every year that marijuana culture is celebrated across the US and beyond, was equipped with a rail system, ventilation, lights and a large elevator, officials said.

The exit on the U.S. side is about three feet wide and was covered by a trash can inside a wooden pallet warehouse.

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Border agents have discovered what could be the longest cross-border tunnel in history, seizing seven tons of marijuana and a ton of cocaine that were crammed inside

The 2,600ft tunnel extended 900ft from a house in Tijuana, Mexico, and across the border for a further 1,500ft

A federal officer wears a tunnel rat logo on the back of his shirt after the discovery of a cross-border tunnel between Tijuana, Mexico to Otay Mesa in San Diego, California

The 2,600ft tunnel extended 900ft from a house in Tijuana, Mexico, and across the border for a further 1,500ft, where it surfaced in a fenced-off industrial area in San Diego

The tunnel, discovered just in time for 4/20, the day every year that marijuana culture is celebrated across the US and beyond, was equipped with a rail system, ventilation, lights and a large elevator, officials said

The huge haul of marijuana and cocaine is one of the largest ever found underneath the U.S..-Mexican border

The entrance to the tunnel on the U.S. side was hidden beneath a trash can in an industrial park and covered with pebbles. The tunnel was equipped with rails and a ventilation system, as well as lights and an elevator from the passage to a closet inside a house on the Mexican side

The exit (pictured) on the U.S. side is about three feet wide and was covered by a trash can inside a wooden pallet warehouse

The tunnel could be the longest ever discovered by border officials.

A man who works near the industrial site told Fox 5 that he had been suspicious of the warehouse for some time. He said the pallet factory had been open for around a year.

'Once I saw them put up cameras inside I knew something was going on,' he said.

'They were here last night and the night before, I came in and was like what's going on and they said they shut everything down.'

Six people were arrested in San Diego and charged with drugs offenses.

The previous longest tunnel found is believed to have been approximately 2,400ft long, slightly shorter than today's discovery.

Border agents have discovered what could be the longest cross-border tunnel (entrance pictured) in history, seizing seven tons of marijuana and a ton of cocaine that were crammed inside

The 2,600ft tunnel (entrance pictured) extended 900ft from a house in Tijuana, Mexico, and across the border for a further 1,500ft

The tunnel was equipped with a rail system, ventilation, lights and a large elevator, officials said. Pictured, agents at the scene

United States Attorney Laura E. Duffy looks down the opening of a hole in the ground after the discovery of a cross-border tunnel from Tijuana, Mexico to the outskirts of San Diego

Border Patrol Special Operations Supervisor Cesar Sotelo looks on next to the entrance of a tunnel leading to Mexico from a warehouse along the border

Meanwhile, another drugs tunnel was found further along the border in Calexico, California, last Friday, by a border agent who almost fell into it.

The official was on a routine patrol two miles east of Calexico, California, last Friday when he spotted an unusual depression in the soil near a canal.

As he walked up to the area, the ground gave way underneath him, exposing the secret tunnel.

The hole was around 18 inches in diameter and had wood and electrical wiring inside, according to U.S. Customs and Border Protection.

The agent, from El Centro Sector Border Patrol, secured the scene ahead of a further investigation.

The tunnel was found to be three by three feet wide and around 142ft in length, the authorities said.

The passage began about 60ft south of the border in Mexico, ending in the United States, around 82ft north of the border.

'This is exactly what our Border Patrol agents are on patrol for,' El Centro Sector Chief Rodney Scott said.

'They are on the lookout for anything out of the ordinary along our vast southwest border. This agent saw something that didn't look quite right and it turned out to be a tunnel.'

A border agent has discovered a 400ft drug tunnel (entrance pictured) in Calexico, California, on the Mexican border by nearly falling into it

The 142ft-long tunnel was found to be three by three feet wide and had wood and electrical wiring inside

The passage began about 60ft south of the border in Mexico, ending around in the United States, around 82ft north of the border. Pictured, agents by the border measuring the tunnel

In another shocking discovery last month, agents found a cross-border passageway - the length of four football fields - filled with more than a ton of marijuana with a street value of $6million.

Drug smugglers had bought a plot of land for $240,000, built a house and then used it to hide a tunnel that led to Mexico.

They finished the house for an additional $86,000, but got the construction workers to leave space for them to excavate.

The owners then dug and joined up the tunnel from the Mexican side which was believed to have been finished by the time they bought the property.

The discoveries demonstrates the enduring appeal of tunnels to smugglers, despite the significant time and money required to build one.

Dozens of tunnels have been found along the U.S.-Mexico border in recent years, some equipped with hydraulic lifts and electric rail cars.

Most of the tunnels found were in California and Arizona, many of them incomplete.

The San Diego-Tijuana region is popular because its clay-like soil is relatively easy to dig with shovels and pneumatic tools, and both sides of the border have warehouses that provide cover for trucks and heavy equipment.

Just in time: The discovery of the tunnel comes just in time for 4/20, the day that marijuana enthusiasts celebrate cannabis culture; here a group of protesters gather at the state capitol in St Paul, Minnesota, calling for an end to cannabis prohibition in the state