Two drivers in San Mateo, Peru were cruising along a highway in the middle of a mountain range ... when, all of a sudden, a rockslide occurred -- with a handful of massive boulders descending at full speed and making contact with the cars down below.

The video's wild -- you see a truck up ahead of the one that's recording from its dashcam, and it gets flattened/tossed by a boulder. Not long after ... the truck recording also gets hit.

Per CNN ... both drivers survived the ordeal, although it's unclear if they sustained injuries.

What makes that even more shocking is when you see what this nightmare looked like from the inside. The driver of the truck with the dash cam footage had another angle showing himself fleeing ... and he narrowly escaped getting squashed, if only by seconds/inches.

There are pics circulating of the aftermath from this ... and it gives you a full sense of how many rocks ended up coming down the mountainside, and how destructive this really was.

As you can see ... the entire highway ended up being blocked by boulders and debris, and first responders were on the scene trying to clear the road as best as they could. According to reports, it took about four hours for them to get the flow of traffic moving again.

As for what may have caused this ... authorities suspect heavy rain possibly destabilized some of the terrain there. That's something folks here in SoCal can relate to these days.