In the Cellars of the Opera

The wall, behind them, having completed the circle which it described
upon itself, closed again; and the two men stood motionless for a
moment, holding their breath.

At last, the Persian decided to make a movement; and Raoul heard him
slip on his knees and feel for something in the dark with his groping
hands. Suddenly, the darkness was made visible by a small dark lantern
and Raoul instinctively stepped backward as though to escape the
scrutiny of a secret enemy. But he soon perceived that the light
belonged to the Persian, whose movements he was closely observing. The
little red disk was turned in every direction and Raoul saw that the
floor, the walls and the ceiling were all formed of planking. It must
have been the ordinary road taken by Erik to reach Christine's
dressing-room and impose upon her innocence. And Raoul, remembering
the Persian's remark, thought that it had been mysteriously constructed
by the ghost himself. Later, he learned that Erik had found, all
prepared for him, a secret passage, long known to himself alone and
contrived at the time of the Paris Commune to allow the jailers to
convey their prisoners straight to the dungeons that had been
constructed for them in the cellars; for the Federates had occupied the
opera-house immediately after the eighteenth of March and had made a
starting-place right at the top for their Mongolfier balloons, which
carried their incendiary proclamations to the departments, and a state
prison right at the bottom.