"You had better stoop a little." She stopped short.
"What is that ?" she asked fearfully.
A sound like rolling thunder, faint at first, but growing more distinct at every step, broke the chill silence of the place.
"The sea," Cecil answered.

"We are getting near to the beach." Jeanne nodded and crept on.

Louder and louder the sound seemed to become, until at last she paused, half terrified.
"Where are we ?" she gasped.

"It sounds as though the sea were right over our heads." Cecil shook his head.
"It is an illusion," he said.

"The sound comes from the air-hole there.
We are forty yards from the cliff still." They crept on, until at last, after a turn in the gallery, they saw a faint glimmering of light.