Right hand panels are a focused view on areas of ice blocks shown in left hand panels. Arrows point to the top of the ice blocks. Thick black marks on the metre stick indicate 10 cm. Panels a and c show ice blocks harvested from boreal lakes with strong ebullition seeps, with close-up view of rough ice bottom (~ 4 cm deep) shown in panels b and d. Panel e and close-up f show an ice block harvested from an ebullition-free gravel pit pond where artificial ebullition was conducted as described in Engram et al.44, demonstrating cavities below gas bubbles are caused by uneven ice growth due to the thermal insulation of gas bubbles. Panel g shows a ‘wedding cake’ pattern of stacked bubbles that had partly vented to the atmosphere. In h, fingers shown for scale point to a crack in the ice, through which gas from the ice pocket periodically escaped. This wedding cake feature is depicted in Extended Data Fig. 2e.