Mexico has now joined Nigeria as a country where corruption is so endemic that it is actually a career-choice in itself. School-leavers apply themselves to the study of this lucrative profession, as others might study medicine or the law. To be a top corrupter is to be king.This is borne out by the life-history of Chapo Guzman, who escaped twice from maximum security jails and was declared the 41st most powerful man on earth by Forbes Magazine. It is into his mountain fastness in remote Sinaloa that the BBC's Katya Adler now ventures, escorted by troops whom we can only hope were genuine. Sinaloa is now the biggest of the cartels, threatening the existence of both Juarez and Tijuana. Adler occasionally turns round to camera with a cheeky comment, just to relieve the grimness of the subject matter, and here she jokes that she can smell they're in cannabis country (now how would she know that?). But in the first bar-room, we're met by loud anthems in praise of folk-hero Chapo, and we realize with a wrench just how far we still are from making any impact on the power of the cartels. For example, when Chapo recently married a teenage bride, the army and police just stood and watched, despite his being the most wanted man in Mexico.There is something a little odd about Adler's cross-lingual interviewing. For one thing, her spoken Spanish sounds to me roughly 'A' level - good enough for Spain, but I've heard that even native Spaniards have trouble exchanging dialogue in Mexico. So I think there must have been some sly rehearsing and editing behind the scenes, to provide the smooth flow of answers that she is able to elicit from the various interviewees, sometimes with the aid of subtitles or dubbing.Adler tries to finish on a note of optimism, but it is hard to summon much hope. She has just escorted a bereaved couple to the place where their son may have been killed and left to rot in a pit of caustic soda, but we just don't know. He'd been abducted by a squad in police uniforms that turned out to be fake. All they have now is the memory of their boy, along with the miserable thought that he may indeed have been drawn into the cartel wars, since the dead and missing are inevitably tainted by association.We end on the graduation ceremony for the latest class of police cadets in Juarez, many of them young women, with nearby El Paso, Texas in sight, but only through the Wall, of course.