I've seen the equivalent of that in gypsy moth (I think) moths when I was a kid. There was a store down the block - because they left lights on, the entire store was covered about 2 nights a year for a couple of years - you couldn't see the windows at all.
But, I've seen far, far worse near water when there's a major hatch. One year my wife and I were out in a secluded bay, doing some fishing after dark. We started hearing a hum that got louder and louder. Could not figure out where the hell it was coming from - it was coming from everywhere. Finally, grabbed a flashlight and aimed it upward. You know how when it's foggy out and your beam extends into the fog? The beam vanished within several feet - thicker than fog with bugs. Got the hell out of there. Turning on the anchor light and navigation lights sucked - started drawing them in.