Have you ever had this unusual occurrence in your freezer? This one observant science teacher says:

We had a single stalagtite form from one cube in an ice cube tray. It rose about an inch, no more than an eighth of an inch in diameter, and tapering to a sharp point. How did that form?

Paul Doherty (physicist extraordinaire) answered that this is called an ice spike.

The water in an ice cube freezes from the outside in. Once the outside is sealed the water inside freezes and expands.
So the interior water is pressurized.
If it freezes at just the right rate the pressure can push the liquid water out of a hole in the top surface and freeze it.

It helps if the water is clean and free of nucleation sites i.e. distilled

See this website here for a bunch more information and great pictures.

Originally posted on my sciencegeekgirl blog.