“Cheek!” exclaimed Ned, and with his merry whistle he hurried off in the darkness. “I only hope Mary speaks to you when you finally get to see her,” floated back to Tom. “Don’t you worry about Mary,” advised the young inventor. “I’ll explain to her. And tell her I’ll be along in about half an hour. I really forgot all about the engagement.” “I’ll say you did!” playfully mocked Ned. Then, with his chum out of the way, Tom gave himself to trying to solve the mystery. For mystery he believed it to be. Seeing a man step behind a bush and, on arriving at the bush, to find nothing of the man there was surprising, to say the least. Sensing that it would soon be so dark that it would be useless to investigate without an illuminant of some sort, Tom made haste to gain what advantage he could from the fast-fading light. He looked sharply about without moving from his place behind the bush on the other side of which he had seen the man disappear. Then, as he could pick up here no clew to the strange happening, the young inventor moved around to the other side. The light was a little better here and Tom saw something that made him fairly gasp with astonishment. He had moved somewhat away from the bush and almost at his feet was an opening in the ground. “This explains it!” murmured Tom, half aloud. “A hole in the ground! He went down there. I knew he couldn’t have dug himself in as quickly as that. But that hole! I never saw it before. It isn’t any of our doing. I’d have known about it if it were.” All the land there belonged to Tom and his father. It was a big field surrounding the fenced-in plant, and often the smooth part of the field was used as a landing place for aeroplanes. Cautiously approaching the opening in the ground and wondering more and more how it had gotten there without his knowledge, Tom saw that it had been closed by some planks placed over it. These were now tossed to one side, as if they had been hurriedly displaced. Scattered about was loose earth which had evidently covered the planks, thus hiding them from the view of a casual observer. “A secret opening!” murmured Tom. “This is certainly the queerest thing I’ve ever seen! What does it mean?” His surprise increased when, as he drew near to the edge of the opening, he saw a rough flight of plank steps going down into the hole. The young man caught his breath sharply, it was so astounding. But with Tom Swift to see and think was to act, and a moment later he began a descent of the steps into the mysterious hole. It might have been the part of discretion to wait until daylight, but a secret opening like this, so near the Swift plant, could mean but one thing, Tom reasoned. “Some one is trying to put up a game on us,” he decided. “Unknown to us he has made a tunnel under our plant. There’s something funny here! I’m going to see what it is.”