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Geographic Constraints of Narco-Tunnels Along the Southwest Border

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Geographic Constraints of Narco-Tunnels Along the Southwest Border

Brenda Fiegel

Geographic Constraints of Narco-Tunnels Along the Southwest Border

Illegal activity is rampant along the US/Mexico border, but seizure and arrest trends help authorities determine what type of activity is most likely in a certain geographic area. For example, narco-tunnels most commonly constructed by the Sinaloa Cartel represent a nearly foolproof method to transport large shipments of drugs, cash, and weapons but they are only used along a limited extension of the international border; namely in Baja California and Sonora. This is because their placement corresponds directly to geographic factors such as soil quality and topography in these areas. Add in logistical components with the geographic factors and it can be determined that tunnels will almost always originate on the Mexican side of the border in cities to include Tijuana, Calexico, San Luis Colorado, or Nogales. Armed with this knowledge, it becomes apparent that the Sinaloa Cartel tunnel construction is restricted despite the fact the shared border extends 1,954 miles and further explains why in recent years, this same group has begun re-using previously seized tunnels.


Without question, Baja California and Sonora are geographic locations of choice to build drug tunnels. In fact, all of the 180 narco-tunnels discovered along the US/Mexico border since 1999 were located somewhere between Tijuana and Nogales with these two cities serving as the epicenters of tunnel activity and this is not by chance.[i] Take for example Tijuana. Of course it is located along the border, but this is not the cities’ sole attribute. Geographically speaking, the soil quality is perfect for digging and there are no mountains or rivers to impede tunnel construction. Logistically speaking, it is close to airports, key highway systems, storage areas, and key marijuana production sites.

Clay Soil

When the summer sun heats the clay-like soil in Baja California, it turns rock hard, but when soft, deep tunnels can be dug underneath the surface.[ii] This means that tunnel construction is most likely to occur during winter months when northern Baja California receives its highest rainfall and temperatures are moderate.  To mitigate flooding during and after construction, tunnels in this area generally follow a very slight uphill gradient which allows for adequate drainage of water. 

In addition to facilitating construction, clay soil also benefits cartels as it inhibits ground-radar penetration sensors commonly used to detect tunnels along the border. Even the most advanced sensors can only penetrate clay soil up to 40ft, but this is not an issue as many of the tunnels in Baja California are 60-80ft underground.[iii]

Highways, Warehouses, and Clandestine Airstrips

Drugs, weapons, and money do not magically appear at the border.  They must first be moved from a point of origin to a point of destination and this is where clandestine airstrips highways, warehouses, and airports come into play.  As reported by El Financiero, the Mexican government has destroyed more than 5,089 clandestine airstrips in the southwest sector of the country since 2006.[iv]  Of these, 789 were located in Ensenada which serves as a key production and receiving center for marijuana shipments.  From Ensenada, Mexican Interstates 1, 3, and 5 provide strategic routes for marijuana shipments moving north.  Once in the US, these routes connect with Interstates 5 and 805 and are utilized to drive large shipments of drugs away from the border area to distribution centers. Storage centers for these shipments are located in Otay Mesa, California and Tijuana. Within Tijuana, tunnels commonly originate in homes and end at warehouses on the US side of the border. The Tijuana area is also located in close proximity to the Tijuana International Airport and the Brown Field Municipal Airport which provides additional logistics support. 

Proximity to Key Marijuana Growing Sites

All of the aforementioned factors are important to ensure successful tunnel operations, but it simply does not make sense to build a tunnel if the product to be moved through is not accessible. In the case of Baja California, key marijuana production sites are located in Ensenada and Rosarito.  This idea is evidenced by the fact that in 2011, soldiers assigned to the II Military Region discovered a 120-hectare marijuana field (one of the largest to ever be discovered by authorities in Mexico) in the Ensenada municipality of Baja California.[v]  The field in question was located in close proximity to Highway 1D which leads up to coast to the US/Mexico border.  In the years since the initial discovery of the mega-field, authorities have continued to make significant marijuana seizures in the area as it serves as a major production center.

Why Nogales?

Nogales shares some of the same characteristics as Tijuana in the sense that it is a border city and is located in close proximity to key highway systems and clandestine landing strips. However, its downward sloping topography into Nogales, Arizona is what really makes this city a prime tunnel center as intense flooding prompted the creation of a US government built drainage culvert in the form of the Grand Tunnel system.  This drainage system extends from Nogales, Arizona into Nogales, Mexico and was originally built in the early 30s to control flooding issues and consists of culverts that lie beneath Morley Avenue and Grand Avenue. These culverts run a full mile into the US and provide unique underground links large enough to drive a car through.  This type of infrastructure is not found anywhere else along the US/Mexico border and the Sinaloa Cartel has taken advantage of it to build smaller, interconnecting passages which feed into the main system.

Flooding Considerations

The Nogales area is dry most of the year, but monsoon season brings significant rainfall between July and September.  During this time, the culverts mentioned in the previous paragraph quickly swell with water currents strong enough to break open floodgates and man-made barriers aimed at preventing drug and human smuggling. For this reason, it is likely that peak trafficking times occur during dry season as does tunnel construction.   

Highways, Clandestine Landing Strips, and Local Housing

Mexican Federal Highway 15 begins at the Mariposa Port of Entry in Nogales, Sonora, and ends in Mexico City. This route also passes through key marijuana production territory in Sonora, Sinaloa, Nayarit, Jalisco, and Michoacan and is of importance as drug shipments are picked up and transported from hub cities along the highway and driven to the border. Once on the US side, Mexican Federal Highway 15 connects directly with Interstate 19 in Arizona which is a short but significant corridor for two reasons.  First, it provides a quick route to Tucson and Phoenix via its interchange with I-10. Second, it forms part of the CANAMEX Corridor which is an important trade route that stretches from Mexico across the US to Alberta, Canada.

Another logistical benefit provided to cartels in Sinaloa are the thousands of clandestine landing strips that have been built to receive incoming drug shipments.  According to Commander Miguel Enrique Vallin Osuna, Sonora represents a prime location to build these airstrips because the climate is dry for most of the year and geographically, it is situated just north of the Golden Triangle which serves as Mexico’s epicenter of marijuana production.[vi] Furthermore, there is little government control in northern Sonora as the aerial radars used to detect illicit flights are located in the southeast sector of the state according to an unnamed Mexican military official.[vii]  This means that cartels have virtually free reign to build airstrips as the only method of detection is through direct observation during reconnaissance flights or anonymous tips from individuals living near them. In terms of storage, Nogales is not an industrial center like Tijuana so it is common for tunnels to connect from drainage systems directly to residential locations or simply from residence to residence.  

A Brief Comparison of Tunnels by Sector

In addition to tunnel placement, geographic factors also dictate the types of tunnels that are most likely to be constructed in a certain sector.  For example, tunnels in the Tijuana sector are many times classified as “super-tunnels” because they exceed the norms of traditional tunnels in the sense that they measure up to 100ft deep, 800ft long, and are lined with re-enforced concrete, rail cars, ventilation systems, security cameras, and elevators.  Tunnels of this nature generally take up to a year to build and cost anywhere from $2,000,000-$3,000,000.[viii]

In comparison, tunnels in the Nogales range from rudimentary to complex and almost always feed off of the main Morely and Grand Avenue tunnel systems. A rudimentary tunnel is characterized by the trafficker having just enough space to push his body through the hole with a small shipment of drugs and costs approximately $30,000.  Once on the US side, a waiting vehicle with a hole built into the bottom is able to receive the shipment at any hour of the day.   The entrance on the US side is usually carefully carved out into the concrete and is undetectable to the untrained eye.  After the drugs have been loaded into waiting vehicles, the concrete opening is closed with a jack. The trafficker then burrows backwards into Mexico as the space of the hole is too small to even turn around. As for complex tunnels, they are similar to those located in Tijuana and may contain rail systems, ventilation systems, lighting, and in some cases, even a water supply. One major difference between complex tunnels in Tijuana and Nogales is that those in Nogales are not generally dug as deeply because flooding issues and the sand soil can result in spontaneous collapse even with concrete reinforcement.  

The Re-Use of Tunnels 

As illustrated by past sections, tunnel placement is strategic and depends on both geographic and logistic factors. These factors also mean that cartels are not simply going to sit back and do nothing if one of their major tunnels are seized which is why they are now re-using sections of previously seized tunnels despite the fact that they have been sealed off and at their point of exit with concrete.  In the case of super-tunnels in Tijuana, open source channels have noted instances in which the Sinaloa Cartel constructs a new tunnel horizontal to the original tunnel effectively connecting the two pathways. At the point of exit, another horizontal tunnel may be dug as close as 100 meters from the original opening.  In this manner, they are able to take advantage of pre-existing infrastructure which greatly cuts down on cost and construction time.  El Universal highlighted two separate cases in which super-tunnels have been re-utilized by the Sinaloa Cartel in Tijuana. This source also noted that the cartel will generally wait anywhere from 1-3 years from the original discovery before re-using a narco tunnel.[ix]

In Nogales, the Grand Tunnel system has been re-used for decades. Here, traffickers are more apt to build small, make-shift tunnels that feed into and out of the main and secondary drainage and sewer systems. Once discovered, the openings are sealed, but this has never dissuaded cartels; likely because they realize that by building new tunnels in close proximity to old ones still requires significant work and investigation by US authorities. This is because the underground environment in the Grand Tunnel system is a no-man’s land for at least three separate reasons.  First, toxic gases such as ammonia and chlorine limit the amount of time that can be spent searching for tunnels.  Second, pitch black darkness cannot even be mitigated with night vision goggles in some parts of the tunnel as they require minimum amounts of ambient light to function.  Third, traffickers are known to line the tunnel walls with sharp objects which poses safety hazards to those who dare go in search of new tunnel systems.  All of these factors combined, in addition to the fact that tunnels can only be built in certain geographic locations along the border mean that re-use of an old system is highly likely.  

In closing, tunnels along the US/Mexico border are unique phenomenons that are not likely to just pop up anywhere.  Their placement is predisposed to geographic factors that encompass soil quality and topography, amongst others.   Logistical factors are also important to the placement of tunnels and should be considered when looking at possible areas of interest.  Finally, because tunnels are only located in specific areas, it is likely that cartels will attempt to re-use them by building interconnecting passageways or simply re-opening them near their point of exit.  To mitigate this likelihood, authorities should continue to monitor the original tunnel and the areas extending from it up to 200 meters.

The views expressed in this report are those of the author and do not necessarily represent the official policy or position of the Department of the Army, Department of Defense, or the U.S. government.

The Foreign Military Studies Office (FMSO) assesses regional military and security issues through open-source media and direct engagement with foreign military and security specialists to advise army leadership on issues of policy and planning critical to the U.S. Army and the wider military community.

End Notes

[i] “Hallado un nuevo ‘narcotúnel’ en la frontera entre México y EE UU (New Narco Tunnel Found Along US/Mexico Border).” El Pais.  Accessed from

[ii] “Tijuana River National Estuarine Research Reserve, California.” The Encyclopedia of Earth.  Accessed from

[iii] “Ground Penetrating Radar Frequently Asked Questions.” GSSR.  Accessed from

[iv] “Sedena destruye al 'Cártel de Sinaloa' 5 mil 89 narcopistas (Sedena Destroys 5,089 Sinaloa Cartel Airstrips).” El Financiero.  Accessed from

[v] “Histórico plantío de marihuana pertenece a 'El Chapo' Guzmán (Largest Ever Marijuana Field Seized in Mexico Belonged to Chapo Guzman).” Univision.  Accessed from

[vi] “Sonora se presta para pistas clandestinas, asegura comandante (Military Commanders Reports that Sonora is Key Location for Clandestine Airstrips).” El Imparcial.  Accessed from

[vii] “Sedena destruye al 'Cártel de Sinaloa' 5 mil 89 narcopistas (Sedena Destroys 5,089 Sinaloa Cartel Airstrips).” El Financiero.  Accessed from

[viii] Taladra Chapo San Diego con 56 tuneles (Chapo Digs 56 Tunnels in San Diego).” El Universal.  Accessed from

[ix] “El Chapo reutiliza narcotuneles (Chapo Re-utilizes Previously Seized Narco Tunnels).”  El Universal.  Accessed from


About the Author(s)

Brenda Fiegel is a Senior Intelligence Analyst and the Editor of the Latin American Operational Environment Watch at the Foreign Military Studies Office (FMSO) in Fort Leavenworth, Kansas. FMSO conducts open-source and foreign collaborative research, focusing on the foreign perspectives of understudied and unconsidered defense and security issues. Her specific research expertise includes “US/Mexico foreign relations,” “US/Mexico border security threats,” “Mexican and Central American violence/extremist groups to include drug cartels” and “Conflict resolution and peacekeeping in Mexico and Central America.” She has lectured on these topics in professional military education settings, at Interagency Security Conferences, at Customs and Border Patrol Facilities, and at academic forums.