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Ice spikes are odd ice structures that occasionally grow out of ice cube trays.  Unlike some of the strange things you might find growing in your refrigerator, ice spikes are made of nothing but ice.  Ice spikes are the result of physics, not biology.

At left are some pictures I took of ice spikes that grew in my kitchen freezer.   They look a lot like the limestone stalagmites found in caves, although there was no water dripping inside my freezer when these formed.

To see your own ice spikes, make ice cubes in an ordinary plastic ice cube tray, in an ordinary household freezer, but using distilled water, which you can buy in most supermarkets for about a dollar a gallon.  We've tried several different freezers, and almost always got some ice spikes to grow.

Ice spikes grow as the water in an ice cube tray turns to ice.  The water first freezes on the top surface, around the edges of what will become the ice cube.  The ice slowly freezes in from the edges, until just a small hole is left unfrozen in the surface.  At the same time, while the surface is freezing, more ice starts to form around the sides of the cube.

Since ice expands as it freezes, the ice freezing below the surface starts to push water up through the hole in the surface ice (see diagram).  If the conditions are just right, then water will be forced out of the hole in the ice and it will freeze into an ice spike, a bit like lava pouring out of a hole in the ground to makes a volcano.  But water does not flow down the sides of a thin spike, so in that way it is different from a volcano.  Rather, the water freezes around the rim of the tube, and thus adds to its length.  The spike can continue growing taller until all the water freezes, cutting off the supply, or until the tube freezes shut.  The tallest spike we've seen growing in an ordinary ice cube tray was 56mm (2.2in) long.

Most ice cube trays produce a few spikes, but usually only if distilled water is used.  Millions of people make ice cubes every day using ordinary tap water, and most don't see ice spikes.  Oddly enough, some people often get plenty of ice spikes using ordinary tap water, but this appears to be rare. 

If you want to read more about the mysterious ice spikes, here are some articles on the subject:

An exploratory study of ice-cube spikes, by C. A. Knight, Journal of Glaciology (2005)

Experiments on ice spikes and a simple growth model, by L. Hill, E. Lozowski, and R. D. Sampson, Journal of Glaciology (2004)

An investigation of laboratory-grown ice spikes, by K. G. Libbrecht and K. Lui, Journal of Glaciology (2004)