Security forces in Mexico have discovered an underground tunnel aimed at crossing into the United States.The unfinished tunnel, in the border city of Tijuana, is believed to have been built by the Sinaloa cartel with the aim of smuggling drugs into the US. Last month Joaquin "Shorty" Guzman, the leader of the Sinaloa cartel, escaped from a maximum security jail through a 1.5km-long (1 mile) tunnel. Guzman, one of the world's most wanted drug dealers, is still on the run.