Okay, true story.
I was boogie boarding off the coast of Kona at a little beach called Disappearing Sands (or Magic Sands) Beach. I was out there, waiting for a good wave, when another guy out there said, "Holy cow!"
I looked all around trying to see what he was talking about, and then something in the water caught my eye. It was a HUGE manta ray swimming right below me. Well, maybe it wasn't freakishly huge, but to me it was freaking huge. Maybe 5-6 feet from wingtip to wingtip.
So that was cool... but that's not the weird part.
About a half hour later, another guy says, "Oh wow!"
I look down into the water expecting to see something else, but I didn't see anything. When I looked around, what I did see was all the surfers and boogie boarders paddling out further into the water. I looked out and there was a HUGE swell coming in. I was in a bad spot for that, so I paddled out as fast as I could.
I made it out far enough and rode over that monster swell. When I was up there, I could see another one coming in and a bunch more behind it.
I watched the big swell turn into a wave. It was BIG. The waves we were riding on beforehand were maybe 4-5 feet. Good size, but not huge. These massive swells were making 10-12 foot waves. The first one broke RIGHT on the sand. People were screaming and running away on the beach.
For the next 20 minutes, huge wave after huge wave kept coming. I was stuck out there with the other guys. There was no way to ride these things since they were breaking on the shore. There was also no way I was going to try to ride a 12 foot wave, no matter where it was breaking.
The guys around me didn't seem too concerned, so I tried not to worry too much about the possibility of being stuck out on the ocean forever.
Then, just as soon as they started, they stopped. The waves went back down to normal. I caught the first one in that I could and got to the beach. Where before it was a nice sandy beach, now about 60% of it was black lava rock. The big waves had eaten away at the beach and the sand had just disappeared! Hence, the name. Probably.