Stalagmites in ice cube tray?!

Factual Questions
1

This is really weird.

We made ice cubes with distilled (Aqua Systems) water in an standard ice cube tray (makes slightly trapezoidal, long cubes).

When we take out the tray, sharp, thin and impressively tall (about the height of the cube or more) projections of ice (like stalagmites) are jutting from several of the cubes.

What could cause this?!

Thanks for your answers.

2

Lotso theories here: Spikecicles (reprise)

3

There really is nothing that we haven’t covered before is there?

4

I shoulda known this had been done before.

Thanks!

5

Real Spikecicle science, with graphs and error-bars and stuff. Cubes made with distilled water, in a fan equipped, -7.5 °C freezer, are most likely to produce ice spikes.