She stood at the bottom of the steps, her torch showing her a narrow tunnel. It curved sharply to the left. “Now will it join the dungeons from here?” she wondered, trying to get her sense of direction to help her. “They can’t be far off. But there’s no sign of them at the moment.”

She went on down the narrow tunnel. Once the roof came down so low she almost had to crawl. She flashed her torch on it. She saw black rock there, which had evidently been too hard to be removed by the tunnel-builders long ago.

The tunnel went on and on and on. George was puzzled. Surely by now she must have gone by all the dungeons! Why — she must be heading towards the shore of the island! How very queer! Didn’t this tunnel join the dungeons then? A little further and she would be under the bed of the sea itself.

The tunnel took a deep slope downwards. More steps appeared, cut roughly from rock. George climbed down them cautiously. Where in the world was she going?

At the bottom of the steps the tunnel seemed to be cut out of solid rock — or else it was a natural passage, not made by man at all. George didn’t know. Her torch showed her black, rocky walls and roof, and her feet stumbled over an irregular rocky path. How she longed for Timmy beside her!

“I must be very deep down,” she thought, pausing to flash her torch round her once more. “Very deep down and very far from the castle! Good gracious — whatever’s that awful noise?”

She listened. She heard a muffled booming and moaning. Was it her father doing one of his experiments? The noise went on and on, a deep, never-ending boom.

“Why — I believe it’s the sea!” said George, amazed. She stood and listened again. “Yes — it is the sea — over my head! I’m under the rocky bed of Kirrin Bay!”

And now poor George did feel a bit scared! She thought of the great waves surging above her, she thought of the restless, moving water scouring the rocky bed over her head, and felt frightened in case the sea should find a way to leak down into her narrow tunnel!

“Now don’t be silly,” she told herself sternly. “This tunnel has been here under the sea-bed for hundreds of years — why should it suddenly become unsafe just when you are in it, George?”

Talking to herself like this, to keep up her spirits, she went on again. It was very queer indeed to think she was walking under the sea. So this was where her father was at work! Under the sea itself.

And then George suddenly remembered something he had said to them all, the first time they had visited him on the island. What was it now? “Oh yes! He said he had to have water above and around him!” said George. “Now I see what he meant! His workroom is somewhere down here — so the sea-water is above him — and it’s all round the tower, because it’s built on an island!”