It takes place in a large hollow tree.

VOA: special.2009.11.27

"little guide to different types of snowflakes sectored plates and capped columns and hollow columns and all different types of crystals.

VOA: standard.2009.12.19

Using a hollow tube, they drill into the tree and extract a 5-mm wide cylinder that shows each ring starting with the most recent, just under the bark.

VOA: standard.2010.04.05