Alcademics

In clear ice cube trays, one cube (or a few) almost always pops up and starts growing upward after some time of freezing. I call this The Mystery Pillar. Others call it the Sacrificial Cube, because it is usually cloudy and must be discarded. 

While it would be theoretically possible to build a deformable tray that avoids this, in general I think you just need to live with it. Pull the tray out of the freezer when it starts forming- if you let it go too long, you may end up with the pillar hitting the freezer ceiling and pinning your tray into the freezer. I say this from experience. 

As water cools and turns into ice, it expands. In a rigid container, this exerts pressure on the system, and it seems that pressure pushes one or more cubes up from the hole in the bottom of the tray. It seems it's the last cube compartment to freeze that becomes the mystery pillar. A sciency video that explains the phenomenon in ice spikes is here.

I think that sometimes the water pushing up into the tray pushes the existing cube up - so that it's clear on top and cloudy down the cube shaft. And other times I think water squeezes up around the cube and onto the top surface so it grows that way - cloudy on the top. 

I've been asked about the Mystery Pillar four times in the past week so going forward I'll point everybody to this post!