Book excerpt: Having followed Mexico's cartels for years, border security expert Sylvia Longmire takes us deep into the heart of their world to witness a dangerous underground that will do whatever it takes to deliver drugs to a willing audience of American consumers. The cartels have grown increasingly bold in recent years, building submarines to move up the coast of Central America and digging elaborate tunnels that both move drugs north and carry cash and U.S. high-powered assault weapons back to fuel the drug war. Channeling her long experience working on border issues, Longmire brings to life the very real threat of Mexican cartels operating not just along the southwest border, but deep inside every corner of the United States. She also offers real solutions to the critical problems facing Mexico and the United States, including programs to deter youth in Mexico from joining the cartels and changing drug laws on both sides of the border.