The U.S./Mexican border was the most crossed, controlled international border in the world, with some 350 million (legal) crossing every year. It spans a length of 1,989 miles and crosses everything from uninhabited dessert to major metropolitan areas.
The border seems to have been a hotbed of activity during the outbreak. According to George Geary, elements of the Mexican Military were deployed to secure the border and to stop the infected (and living refugees) from crossing into Mexico from America. At the time Madison's Group cross into Mexican waters aboard The Abigail, only elements of the Mexican Navy were operational in Baja California.
According to Sierra, the border was a nexus of activity, with people fleeing both south and north in order to escape the Infected. Sierra's traveling companion Gabi claims to know a safe haven in Mexico, and both Gabi and Sierra head in the direction of the boarder. According to Gabi, her boyfriend Colton knows about a tunnel under the border, which is the intended destination for Gabi and Sierra. Once they reach the tunnel, it is apparent that it was in the process of being filled in as it is situated within the confines of the Construction Site. It is here that they meet Colton. Colton tries to stop Sierra from descending, but she overpowers him. It turns out that the tunnel was booby-trapped.