The passage seemed to run down one side of the building, but the vaults that gave on to it led each one into another, so that the place was something of a labyrinth. The knocking sounded distinctly now, echoing through the empty cellars. Charles held his torch lowered, so that the circle of light was thrown barely a yard in front of him.
Suddenly the knocking ceased, and at once both men stood still, waiting for some sound to guide them.
Ahead of them, where the passage ended, something moved. Charles flashed his torch upwards, and for a brief instant he and Peter caught a glimpse of a vague figure. Then, as though it had melted into the wall, it was gone, and a wail as of a soul in torment seemed to fill the entire place.
The sweat broke out on both men's foreheads, and for a second neither could move for sheer horror. Then Charles pulled himself together and dashed forward, shouting to Peter to follow.
"My God, what was it?" Peter gasped.
"The groan we've all heard, of course. Damn it, he can't have got away!"
But the place where the figure had stood was quite empty. An embrasure in the wall seemed to mark the spot where they had seen it, yet if the apparent melting into the wall had been no more than a drawing back into this niche that could not solve the complete disappearance of the figure.
The two men stared at one another. Charles passed the back of his hand across his forehead. "But - but I saw it!" he stammered.
"So did I," Peter said roughly. "Good God, it can't be… This is getting a bit too weird to be pleasant. Look here… Damn it, that was no ghost. There must be a secret way through the wall." His torch played over the wall. It was built of great stone slabs each about four foot square. He began to feel them in turn. "We must be under the terrace," he said. "Gosh, don't you see? We're standing on the level of the ground here!" One of the blocks gave slightly under the thrust of his hand. "Got it!" he panted, and set his shoulder to it. It swung slowly outward, turning on some hidden pivot, and as it moved that hideous wail once more rent the stillness.
"So that's it, is it?" Charles said grimly. "Well, I don't mind telling you that I'm damned glad we've solved the origin of that ghastly noise." He squeezed through the opening in Peter's wake, and found himself, as Peter had prophesied, in the garden directly beneath the terrace. There was no sign of anyone amongst the shrubs near at hand, and it was obviously useless to search the grounds. After a moment both men slipped back into the cellar, and pushed the stone into place again.