As drug-trafficking-related violence in Mexico raged, the cartels came up with a radical solution for improving their capabilities in face-offs with other criminal groups and Mexican security services.

Narco tanks are homemade armored vehicles, also known in Spanish as “monstruo” for their hulking size. They reached peak popularity in 2011 as the Mexican military seized a garage from the Zetas drug-trafficking organization that was being used to construct the vehicles. At that point, criminal gangs could operate military-like vehicles out in the open with apparent impunity.

The Mexican military’s subsequent crackdown on the creation of monstrous forced the practice even further underground. Narco tanks are still produced, but today’s versions have their armored paneling on the inside so as to not draw unwanted attention from rival cartels and the military.

In January, Mexican authorities surrounded the hideout of notorious drug lord Joaquín “El Chapo” Guzmán during the Black Swan operation, the raid that eventually led to Guzmán’srecapture.

Mexican marines confiscated weapons and two narco tanks from Guzmán’s hideout.

Below is a breakdown of the various features that made narco tanks into seemingly impregnable, drug-running beasts.