After the bell-, puppet- and horsemen-ice, it's now the turn of the cipher-ice.
It doesn't consist of bubbles, as would seem likely, but has an amorphous structure. Presumably it's subsoil water that has taken in gasses and minerals during its rise through the strata.
Consequently it's water of a different character and temperature. It got frozen into the rest of the ice, thus forming a misleading cipher.