It was immovable.

Again he thrust his hand and arm through the first of the narrow ventilating apertures.

The wood with which his fingers came in contact was rotting from moisture and age and he found that he could tear out handfuls of it.

He fell to work, digging with the fierce eagerness of an animal.

At the rate the soft pulpy wood gave way he could win his freedom long before the earliest risers at the post were awake.
A sound stopped him, a hollow cough from out of the blackness beyond the dungeon wall.