They are known in the U.S. Border Patrol as "tunnel rats" — agents who go in clandestine passages that have proliferated on the U.S.-Mexico border over the past 20 years to smuggle drugs.
The Associated Press joined the Border Tunnel Entry Team, as it is formally known, inside an incomplete tunnel that was discovered in San Diego in 2009 — 70 feet deep, 3 feet wide, 2,700 feet long and equipped with a rail system, lighting and ventilation.
Here are some questions and answers about the team's work:
HOW MANY TUNNELS ARE THERE?
Authorities discovered 224 border tunnels originating in Mexico from 1990 to March 2016, including 185 that entered the United...