The skies above us might look normal the vast majority of the time, but every so often you'll look up and see something gobsmacking.
For most of us that might just mean a really great sunrise or sunset, or with enough planning perhaps even a solar eclipse, but a video uploaded to Reddit showed something more unnerving.
The user posted a quick clip of a cloud formation with a pretty immediately accurate title: "Clouds look like an incoming tsunami."
The clouds do indeed look like giant waves, with the dark blue of the stormy sky around them punctuated by what seem to be the crests of those enormous waves.
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It's quite astounding to look at, and your eyes can easily be deceived if you just glance at the footage.
But in fact, the cloud formation in question is quite well known and is called arcus, or roll cloud, although it's often a smoother and more unified formation than this clip shows.
Unsurprisingly, though, the post went viral when it was uploaded a couple of years ago - with more than 40,000 upvotes at the time of writing, and hundreds of comments.
These are pretty uniformly from people in awe at the sight of the clouds, with the top comment reading: "If I looked out my window and saw this I would have a heart attack".
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It's quite hard to argue with that - if you knew your clouds, you might know what you're looking at, but most of us would probably be a little shocked by this sight.
Another user referenced the popular movie Interstellar by writing: "'Those aren’t mountains, those are waves'".
This brings up the moment in that movie when Matthew McConaughey and his colleagues, on a waterlogged alien world, suddenly realize that a distant front of mountains is getting nearer - and is, in fact, a massive tsunami.
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It's a great callback because the giant waves in the film do look really similar to the cloud formation in the post.
Still, as always on the internet, there are some others in the thread of comments who maintain the sight isn't anything too special. One person said, matter-of-fact: "You see them pretty frequently in the midwest. It's just a front. I think they started calling them derechos. The front hits with really high winds then there are storms. Often tornadoes behind them. You know they are about to hit because the sky will turn orange and start hailing".
While that might be a pretty factual response, at least you can add the word 'derecho' to your vocabulary in case you ever see a front like this for yourself.