Science has taught us so much about the world and has led to so many technological innovations, that when you step back and consider it all, it’s sort of mind-blowing. But science has its limits, too.

For every answer it gives us, there’s a problem that leaves scientists stumped. Check out some of these crazy natural phenomena that are still huge question marks in the science books…

1. Scientists have consistently observed strange flashes of light that precede volcanic eruptions. They suspect the coming eruption shifts electrical charges in the surrounding geological formations, but that hypothesis has yet to be conclusively proven.