They’re called Ice Spikes and [there’s a whole wikipedia page about it.](https://en.wikipedia.org/wiki/Ice_spike)
Basically:
* The top of water freezes first (because that’s what’s exposed to the freezer, plus ice floats on liquid water), EXCEPT
* Water expands as it freezes
So the surface gets almost completely frozen over, and then as the inside of the cube starts to freeze, the expansion has nowhere to go except out the remaining hole in the ice on the surface. So it forces water up and out the small hole, where of course it cools and freezes, generating the spike.
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