“Have you ever seen any hint of such a hidden chamber, Mr. McNab?” the professor said.

“No, I ha’n’t,” Rory snapped. “Poppycock!”

The professor looked out of a window at the small pond and dark trees. Suddenly he turned, his eyes bright.

“By George, I think Pete’s right!” he cried. The Scottish Highlands are full of hidden caves and caverns. Mrs. Gunn, the letter says to remember what Angus loved in Scotland, but you don’t know what it was. What if it was — ”

“A secret underground cave where they used to meet when they were younger!” Jupiter said. “Something only Laura would know!”

“Which Angus reproduced here,” Professor Shay went on. “What he brought from Cabrillo Island could have been old Spanish furniture and rugs for the hidden cavern!”

“And a mirror, too!” Bob added.

Professor Shay nodded eagerly. “I think we have it, boys! It’s obviously well hidden, and the entrance is probably covered over after a hundred years. But we’ll find it! First thing tomorrow we’ll start combing every inch of Phantom Lake!”

“Why not tonight!” Pete exclaimed. “We’ve got lights.”

Professor Shay shook his head. “I’m sure we couldn’t find much in the dark. Besides, we’re all tired. We’ll be more alert after a good night’s sleep.”

“The treasure won’t run away, boys,” Mrs. Gunn said firmly. “Cluny, for one, is going to bed right now.”

“But we know Stebbins is hanging around,” Cluny protested, “and probably Java Jim, too!”

“I doubt they’ll find much at night, either,” Professor Shay said. “We’ll have to risk it, but I don’t believe it’s much of a risk, boys.”

They all nodded glumly. They knew the professor was right, but it would be a long night of waiting.

“I’ve got a hunch we won’t sleep very well,” said Pete.

“Then think about every possible way an underground chamber could be hidden,” the professor said, “and tomorrow we’ll all gather here and start searching.”

“Ye’ll search wi’out me,” Rory said flatly. “I’ll ha’ no more o’ this nonsense.”

Professor Shay drove away in his station wagon. Pete, Bob, and Jupiter helped Hans load the pieces of furniture that Mrs. Gunn had picked out for Uncle Titus, and then climbed into the back of the truck. The big Bavarian drove to the highway and turned towards Rocky Beach.

For a time, the boys rode in silence. Then Jupiter asked, “How would someone mark an underground room, fellows?”

Pete thought. “Maybe pile up some of those big stones to look natural — but give Laura a clue?”

“Or,” Bob said, “maybe plant a tree? A special tree like one they had at home in Scotland?”

“Yes,” Jupiter said, “that’s possible, Records.”

“Maybe a mirror!” Pete exclaimed. “On the ground, or in a tree, and Laura would see it from some special spot!”

“From a window where she sat at home,” Jupiter said. “Or from the top of the lodge’s tower!”

“Gosh,” Bob said, “any of those would work! I’ll bet one of them is right, Jupe!”

Jupiter nodded, and stared out of the back of the truck at the first houses of Rocky Beach.

“Only one thing bothers me a little,” the Leader of the trio said slowly. “Old Angus’s letter said to remember the secret of Phantom Lake — the phantom that watches for enemies coming up the lake. A hidden cavern doesn’t seem to fit in with that legend.”

“Maybe when we find the cavern we’ll make the connection,” Pete said.

“Yes, perhaps you’re right, Second,” Jupiter agreed.

Hans dropped Bob and Pete at their homes and drove on to the salvage yard. When Jupe got home, he was too excited to sleep right away. He had some hot chocolate and told Aunt Mathilda and Uncle Titus of the day’s adventures. Uncle Titus hurried straight out to inspect what Mrs. Gunn had sent him. Aunt Mathilda decided that a big hole hidden underground sounded exactly right to fit the riddle.

“You’ll find it in the morning, I’m sure,” she said. “Now I want you in bed, young man. You’ll think much better when you’re rested. Off with you!”

Jupiter lay awake for a long time watching the Christmas lights of Rocky Beach. But at last he fell asleep, still thinking of the hidden room, the big stones, the sluice timber, and Cabrillo Island where old Angus had gone to get…

Jupiter sat bolt upright in his bed!

He blinked, still half asleep. It was dark outside his window, but his dock showed that it was almost 8.00 a.m. Then he heard the drumming on the roof, and realised that it was raining hard outside.

But he didn’t think now about the rain.

He sat there and stared at the wall. He knew the whole answer to Angus Gunn’s riddle!

Jupiter dressed and called Bob and Pete. He told them to meet him at the salvage yard in fifteen minutes. He had the answer!

“I’ve been dumb,” the stout leader of the trio moaned. “I should have seen it long ago. Hurry!”

He called Cluny at Phantom Lake.

“I think I know where the treasure is, Cluny,” Jupiter declared to the sleepy boy on the other end of the phone. “Get a pick and shovel and your raincoat, and wait for us. Hans will drive us out,”

He hurried downstairs to have a quick bowl of cereal. As he gulped his milk, the telephone rang. It was Professor Shay.

“Jupiter?” the professor said. “I’ve been lying awake in bed thinking about our hidden room, and I’ve had an idea of how old Angus could have marked it! The phantom — ”

“There isn’t any hidden room. Professor,” Jupiter told him. “I’m certain I know the answer now!”

“What?” Professor Shay cried over the telephone. “Not a hidden room? Then…? Tell me, Jupiter!”

“I’ll tell you at the lake. Meet us there.”

“I’ll get dressed at once!” the professor said.

Ten minutes later, The Three Investigators huddled in the rain in the salvage yard. Pete and Bob could barely contain themselves. When Hans arrived with the truck, they clambered into the covered back and faced Jupiter.

“What is the answer. First!” Bob demanded.

“Tell us!” Pete echoed.

“All right,” Jupiter said, with a maddening grin. “I was asleep, and the hidden room theory was bothering me, and something Bob said when we rode home must have popped into my head. Then I saw the whole thing!”

Pete groaned in the bumping truck. “What did Bob say?”

“He said,” Jupiter intoned solemnly with his love of drama, “that maybe old Angus planted a special tree at Phantom Lake. And that’s exactly what Angus did!”

“A tree?” Pete gaped.

“Not a tree that he knew from Scotland, as Bob thought,” Jupe went on, “but a tree that would make Laura think of home. He went to Cabrillo Island and bought one of those twisted cypress trees that look like phantoms! He planted a phantom at Phantom Lake!”

“Wow!” cried Bob. “All we have to do is find an old cypress out at Phantom Lake!”

“But,” Pete objected, “where do we look? There’s acres and acres of trees out there.”

“The rest of the riddle tells us.” Jupiter beamed. “Think of the steps to the puzzle again. First, the miners and the sluice timber from Powder Gulch. Pete was absolutely correct — miners dig best, and they did dig a big hole. And there is one vital fact about sluice timber we completely overlooked. Why did old Angus have to have sluice timber? Not just planks, or mining timbers, but sluice timber?”

“Why, Jupe?” Pete sighed.

“Because sluice timber is specially cut and fitted to hold water!” Jupiter declared. “A sluice holds water in, but old Angus used it to hold water out!”

Bob stared. “Out of where, Jupe?”

“Out of the big, long hole he had the miners dig for him,” Jupiter said. “He had to keep water out of the hole while it was being dug. Then he bought ten large stones to use as stepping stones. He got a cypress from Cabrillo Island. And what he bought at Wright and Sons was a ship’s lantern!”