A horrifying accident just went down in northern Alabama -- with a combat helicopter falling out of the sky and landing near a freeway ... and the whole thing's on video.

The tragedy occurred Wednesday afternoon in Huntsville, where eyewitnesses say that the military-grade chopper started to descend toward the ground out of nowhere ... only to slam and burst into flames along the median of Highway 53.

Fortunately, it doesn't appear any civilians were harmed in the crash -- but the Madison County Sheriff's Office says sadly, everyone onboard perished ... it's unclear how many exactly. Here's another thing, officials are now confirming this was, in fact, a Black Hawk -- which are normally designated for war zones. Some reports suggest it was a UH-60 model.

The moment the helicopter started to descend was partially caught on a Nest home security camera -- which actually caught the moment it collided as well ... which you can hear in scary detail. This was filmed from quite a distance, but it's incredibly jarring to watch.

Aftermath footage taken from a closer POV is just as distressing ... massive plumes of black smoke erupted from the scene -- and first responders were rushing to attend to the fire.

So far, there's no explanation for why a Black Hawk was in the area or how/why it crashed. An investigation is underway, and cleanup efforts have blocked off all surrounding traffic.