A lacuna is defined in the geologic dictionary as follows:
1. A chronological stratigraphic unit representing a gap in the record. Syn: unconformity.
2. A pore, opening or hole, or gap in various invertebrate organisms.
For those looking to fill a gap in the stratigraphic/evolutionary record, please be advised that a glacial lacuna is not a "various invertebrate organism"...
No. Far from it. A glacial lacuna is actually, through a rough translation, a lake. Now, when I was standing in this particular lacuna, on the Piedmont lobe of Bering, I can certainly say the only liquid water here was a small ephemeral stream running from the top of the ice into this very deep void.
It is thought that these formations are due to rapid wasting of ice in certain locations, but not through typical phase change. What makes the most sense to this particular, amateur glaciologist is sublimation, when solid water transitions to gaseous water. Believe it or not, the temperature on the top of the ice sheet this day was a comfortable 55F or so, while within the lacuna, out of the Chugach Range's strong adiabatic wind, was over 75F.