I used to wonder this as well, until I got to watch a glacier break off into the sea. It moved as slowly as anything on a nature documentary. It’s an issue of scale, and perception. There was really nothing around for me to visually judge how far I was from it, and how fast it was travelling. No landmarks other than “More ice”. It’s a really huge thing, accellerating into the water, but it looks slow because I have no sense of scale. Side note, the MASSIVE splash also looked slow in rising and then falling back into the water because I had nothing to judge it against. Just more water, and a big chunk of ice.
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