N_Cook wrote:

Pool freezing from outside to centre and ice expanding so free water pushed
up through the last remaining central hole until all frozen ?

This is correct. You can achieve a similar thing using distilled water
in an ice cube tray in a normal kitchen freezer. The water has to be
quite pure (or you have to be quite lucky) as impurities in the water
can block up the "growth".

There was something in New Scientist about this some time in the last
year or two!

Jonathan