It went down Monday in the Mexican city of Jala, right on the edge of a mountain bordering the Tepic-Guadalajara Highway between Chapalilla and Ixtlan del Rio. It's unclear what set off the explosion, but it was big enough to stop a bunch of drivers dead in their tracks.

One video circulating right now shows a couple guys running toward what appears to have been an initial discharge that caused traffic to come to a screeching halt. As they approach, an even bigger explosion goes off ... sending a massive fireball into the air and everyone running for their lives.

Another angle shows the aftermath from a further distance, and it becomes clear pretty quickly just how massive this thing was. There's plumes of black smoke emanating from every which way as the flames rage. It's a terrifying sight, to say the least.

There are different reports on the official body count -- we're seeing estimates from 6 to 14 thus far, plus several other injuries and word of missing people as well. There's also extensive damage to numerous cars that were caught up in the explosion.

Check out this shot of the aftermath -- an entire vehicle is lodged into the side of the charred mountain, and car parts are scattered about everywhere. Pieces of the concrete barrier were blown to smithereens ... demonstrating the ferocity of the impact.