Pictured: a look inside alleged Tijuana drug smuggling tunnels after police find one too long to explore without oxygen tanks

Mexican authorities found last week a suspected drug-smuggling tunnel in Tijuana so deep that they haven't been able to find the exit point due to a lack of oxygen tanks.

While the discovery garnered attention for being the first of the year, other similar constructions have been found by law enforcement throughout the years, some of them also featuring similar lengths and depths. In fact, close to 20 tunnels have been found on California's border with Mexico since 2006, according to CBS News.

In January 2020, federal agents found what was then described as the longest drug-smuggling tunnel between Mexico and the U.S. Stretching about three-quarters of a mile, and 70 feet under the ground it also started in Tijuana, the exit point being in San Diego.

The tunnel was equipped with air ventilation, electricity, a rail and cart system and an elevator at both entrances, U.S. Customs and Border Protection said back then. Then-Drug Enforcement Administration's Special Agent in Charge John W. Callery said its "sophistication" showed "the determination and monetary resources of the cartels."

Mexican authorities found a similar tunnel two years later. They located its starting point in Tijuana but said it was of a lower-caliber, as it had a 20-foot depth and 60-foot length. Its exit point was an warehouse in an industrial area near San Diego's Otay Mesa border crossing. Six people found there, all southern California residents, were charged with conspiring to distribute cocaine.

Another discovery was made the following year, back then at a depth of 90 feet. However, since it was 60 feet long when found authorities couldn't conclude whether it sought to run all the way to the U.S. or connect with another one as part of a network.

Law enforcement officials have not confirmed the length or depth of the tunnel as they haven't fully explored it yet. Baja California's Secretary of Public Safety, Leopoldo Tizoc Aguilar Durán, said his team is in the process of getting oxygen tanks to finish the investigation.

Aguilar did confirm that the tunnel is connected to another one found two years ago that does run into U.S. territory. American authorities have been notified of the finding and are looking for an exit point in the country, he said.

Neither American nor Mexican officials have confirmed the tunnel was used to smuggle drugs. In accordance with federal law, U.S. authorities must fill the local side with concrete after tunnels of the kind are discovered.