As a border state, California has a higher incidence of drug trafficking from outside the country than inland states. Because of this, it requires more intensive patrolling and law enforcement actions to keep drugs out of the country.
According to USA Today, the points where drugs come into the country are also very dangerous. Shoot outs are not uncommon.
Many traffickers will come into the state on foot. They come through the canyons. While there have been walls going up to help stop this, they do not deter everyone. The walls, at most, help border patrol to maintain more control. They do not stop the drugs coming into the country.
Underground tunnels are not just a Hollywood creation. Traffickers really do use tunnels dug under the ground to bring drugs and other things into the country. These entry points are incredibly difficult to discover and even harder to stop.
Rivers and waterways
Another point of entry is over rivers. Traffickers will carry the bundles and work through the water to get into the U.S. Other water entry points along the coast provide more possibilities for smuggling drugs into the country.
Perhaps the most surprising point of entry is on the roads. People come into the country in legal ways, but carrying illegal cargo. According to The Washington Post, this is the most common way for traffickers to enter the country.
Traffickers are incredibly creative, finding a variety of ways to circumvent discovery of their illegal cargo. They know the methods border patrol agents use and prepare for them.