The first discovery that they made was that the oaken panels of the chamber were backed with sheet iron or steel. "It is a hopeless job; we shall have to get another kind of smith here to tear down all the panellings," said T. B., lighting the gloom of his despair with a little flash of humour. He fingered the tiny locket absently and opened it again. "It is absurd," he laughed helplessly. "Here is the solution in these simple words, and yet we brainy folk from the Yard cannot understand them!" "God sav the Keng!" said Ela ruefully. "I wonder how on earth that is going to help us." A gasp from T. B. made him turn his face to his chief. T. B. Smith was pointing at the piano. In two strides he was across the room, and sitting on the stool he lifted the cover and struck a chord. The instrument sounded a little flat and apparently had not received the attention of a tuner for some time. "I am going to play 'God save the King,'" said T. B. with a light in his eyes, "and I think something is going to happen." Slowly he pounded forth the familiar tune; from beginning to end he played it, and when he had finished he looked at Ela. "Try it in another key," suggested Ela, and again T. B. played the anthem. He was nearing the last few bars when there was a click and he leapt up. One long panel had disappeared from the side of the wall. For a moment the two men looked at one another. They were alone in the house, although a policeman was within call. The main force was gathered in the vicinity of the Secret House. T. B. flashed the light of his indispensable and inseparable little electric lamp into the dark interior. "I will go in first and see what happens," he said. "I think we will both go together," said Ela grimly. "There is a switch here," said T. B. He pulled it down and a small lamp glowed, illuminating a tiny lift cage. "And here I presume are the necessary controlling buttons," said T. B., pointing to a number of white discs; "we will try this one." He pressed the button and instantly the cage began to fall. It came to a standstill after a while and the men stepped out. "Part of the old working," said T. B.; "a very ingenious idea." He flashed his lamp over the walls to find the electrical connection. They were here, as they were at the other end, perfectly accessible. An instant later the long corridor was lighted up. "By heavens," said T. B. admiringly, "they have even got an underground tramway; look here!" At this tiny terminus there were two branches of rails and a car was in waiting. A few minutes later T. B. Smith had reached the other end of the mine gallery and was seeking the second elevator.