The gore. Oh my God, the gore.

“Piranha 3D” is one of the bloodiest, most disgusting horror films ever made, and that’s saying something in the age of “Saw,” “Hostel” and the numerous other exploitative 80s-era remakes. “Piranha” is also, of course, presented in 3D, with various severed body parts constantly being tossed at the screen.Normally, sitting through this type of movie makes me want to quit the fledgling horror genre altogether. But “Piranha 3D” is so deliberately cheesy and self-aware that it’s impossible not to cackle with sick amusement every time a snotty hard body stumbles into the feeding frenzy.

“Piranha 3D” delivers exactly as advertised – Blood, unnecessary nudity, clunky dialogue, cheesy 3D stunts and CGI monsters. Consider it the “Snakes on a Plane” -caliber guilty pleasure of 2010.

The plot needs very little explanation. An underwater tremor releases thousands of prehistoric piranhas into an Arizona Spring Break hot spot. They love human flesh and can chomp a bikini-clad babe into a skeleton in 3.6 seconds flat. Various objects fly out at the audience in 3D, from innocent seaweed to graphically-severed private parts (Just take a bathroom break when the porn director (Jerry O’Connell) finally falls into the water).

There will be viewers who will honestly consider “Piranha” the worst movie ever produced. I’d argue the bad parts are deliberately bad, and that director Alexandre Aja handles the lackluster material with twisted expertise. How else to explain the presence of respected actress Elisabeth Shue as the noble town sheriff? Or Richard Dreyfuss playing a character who looks and sounds suspiciously like his “Jaws” character?

This summer’s 3D offerings have been a far cry from “Avatar,” so it’s nice to see “Piranha 3D” actually utilize the format for cheap, B-movie tricks. Here’s a movie that makes no apologies for exploiting the trendy gimmick.

The 3D guffaws are half the cheesy fun. Much like “Snakes on a Plane,” “Piranha 3D” works best when experienced with a group who understands the cheesy references and can laugh at the all cartoonish blood-spilling. Those who take it too seriously or see it on video three months from now will just be confused.

Bottom line: Any movie with Christopher Lloyd as the hysterical scientist and Ving Rhames as a trigger-happy cop must be considered essential theatrical viewing. And if that statement doesn’t make sense to you, then do yourself a favor and just stay home.

Grade: B