It was difficult to walk there, because the slope of the hill was so steep. Timmy went with them, excited at this unexpected walk.

“Now, Timmy listen — we want you to show us how you got in,” said George. “Are you listening, Timmy? Go in, Timmy, go in where you went this morning.”

Timmy waved his long tail, panted, and let his tongue hang out in the way he did when he wanted to show he was being as helpful as he could. He ran in front, sniffing.

Then he suddenly stopped and looked back. He gave a little whine. The others hurried to him.

The moon most annoyingly went behind a cloud. Julian took out his torch and shone it where Timmy stood. The dog stood there, looking very pleased.

“Well, what is there to be pleased about, Timmy?” said Julian, puzzled. “There’s no hole there — nowhere you could possibly have got in. What are you trying to show us?”

Timmy gave a little bark. Then suddenly leapt about four feet up the uneven stones of the wall, and disappeared!

“Hie — where’s he gone?” said Julian, startled, and flashed his torch up. “I say, look! There’s a stone missing up there, quite a big block — and Timmy’s gone in at the hole.”

“There’s the block — fallen down the hillside,” said Dick, pointing to a big white stone, roughly square in shape. “But how has Timmy gone in, Ju? This wall is frightfully thick, and even if one stone falls out, there must be plenty more behind!”

Julian climbed up. He came to the space where the great fallen stone had been and flashed his torch there. “I say — this is interesting!” he called. “The wall is hollow just here. Timmy’s gone into the hollow!”

At once a surge of excitement went through the whole lot. “Can we get in and follow Timmy?” called George. “Shout to him, Julian, and see where he is.”

Julian called into the hollow. “Timmy! Timmy, where are you?”

A distant, rather muffled bark answered him, and then Timmy’s eyes suddenly gleamed up at Julian. The dog was standing down in the small hollow behind the fallen stone. “He’s here,” called back Julian. “I tell you what I think we’ve hit on. When this enormous wall was built, a space was left inside — either to save stones, or to make a hidden passage, I don’t know which. And that fallen stone has exposed a bit of the hollow. Shall we explore?”

“Oh, yes,” came the answer at once. Julian climbed down into the middle of the wall. He flashed his torch into the space he was standing in. “Yes,” he called, “it’s a kind of passage. It’s small, though. We’ll have to bend almost double to get along it. Anne, you come next, then I can help you.”

“Will the air be all right?” called Dick into the passage.