“Gomorrah” is one of the most depressing films I’ve seen in quite some time, but it is also one of the most powerful — a brilliant and fervently downbeat exposé of the Camorra, the mobsters who dominate the Italian city of Naples.

It’s based on the non-fiction book of the same name by Roberto Saviano. The book caused such a sensation in Italy that Saviano instantly had a multitude of hits taken out on him. Watching the film, it’s easy to understand why the mob was upset. “Gomorrah” feels as if it’s been made by people with an intimate understanding of the many-headed hydra that has eaten Naples up. Unlike its more famous Sicilian counterpart, this mob does not involve any families or traditions, but rather a bewildering array of constantly shifting gangs and clans.

Accordingly, the film splits off into different strands. A few of them are driven by pretty straightforward mob violence: There are two young men with dreams of Scarface glory who run afoul of the mob after they come across a hidden cache of weapons, and a 13-year-old who joins a gang after returning drugs and guns to them (there’s a shot of mobsters handing out drugs like raffle tickets to a begging crowd).

But we also get a sense of how deeply and exotically the Camorra has insinuated itself into every sector of the economy. Who knew, for instance, that making dresses was a mob-run job, as a garment worker finds out? Or that there’s a deadly market in the illegal dumping of toxic waste, as a grad student discovers when he starts working for a boss who makes money while he poisons people?

The principal characters are connected only because they’re all completely in over their heads. Whether driven by macho fantasy, poverty or ignorance, they quickly discover that there’s no way out, and that their bark won’t prevent them from the mob’s bite. Not to give anything away, but things don’t turn out too well. This isn’t a happy film.

The palette is as moody and insular as “The Godfather” — when kids first join the Camorra, we see them literally stepping into blackness that swallows them whole — but it contains none of that film’s rich opulence. Instead, we get the darkness and decay of the crumbling, labyrinthine concrete monstrosities that are the projects in Naples. The film reeks of heat and sweat and blood.

Similarly, these gangsters are not seductive or romantic. They’re just bastards and criminals, and the revulsion that permeates “Gomorrah” is very potent. The central characters are all sympathetic, though. The dressmaker, Pasquale (Salvatore Cantalupo), is only trying to make more money when he agrees to do a late-night, secret fashion tutorial for a Chinese garment factory — a big mistake. (This isn’t your grandfather’s Italy either; the sizable immigrant population plays a big role here.) And the young people have become so immune to, and entranced by, the power of the mob that, the film suggests, there isn’t much hope for them in the long run: everyone in Naples eventually encounters the Camorra.

The polemical nature of the film sometimes brings “Gomorrah” dangerously close to preachiness, but it avoids this because of the sharpness with which it’s made and because it’s describing something that really is frightening. There’s no poetry in its violence and no glamour in its high life — just a straightforward and courageous kind of despair. The fact that it wasn’t nominated for an Oscar is yet more proof of the Academy’s insanity.