Then, when the beam of the flashlight was cast on the dense covert of branches, the mystery was clear. Beyond the bushes was a dark opening in the rock. “A cave!” exclaimed Frank, in a suppressed tone. It was so cleverly concealed that it could not have been seen in the clear light of the day except at close quarters. The glare of the flashlight, however, cast the dark opening into prominence behind the screen of leaves. This, then, was the explanation of the boat’s disappearance. There was a channel in the cove enabling the smugglers to row the boat directly into this cave in the rock. This also probably explained the presence of the motorboat. “They went in here,” said Joe. “We’ll explore it.” Having gone so far, there was no going back. The boys were fully determined to keep on the track of the smugglers. They did not know what lay behind the darkness of that silent and mysterious opening in the rock. But they meant to find out, no matter what the risks. Cautiously, they advanced into the bushes, which gave way protestingly before them. The branches whipped their faces. The water was still shallow, for there was a narrow ledge along the side of the channel and, moreover, it was now low tide. At last the bushes closed behind them. The Hardy boys were standing in the entrance to a secret passage, pressed close against the rocky wall of the cave. CHAPTER XVII THE CHAMBER IN THE CLIFF Frank switched on the flashlight. The beam illuminated the depths of the dark passage. Far ahead of the brothers they glimpsed a grey shape just above the surface of the glistening water. For a moment they were startled, then they recognized that the grey shape was nothing more than the rowboat that had passed by them in the darkness outside the cove. It had been drawn up close to a natural wharf hewn out of the solid rock. It swayed to and fro with the motion of the water. The boys made their way forward along the ledge, which was wide enough for one person to walk on, until at last the ledge widened out and proved to be a path leading to the wharf. There was not a sound in the passage but the drip-drip of water from the gloomy walls. The Hardy boys stole quietly forward along the wharf, passed the boat, and then looked about them. Frank played the beam of the flashlight all about the place until at last the glare revealed a dark opening immediately ahead. It was a crude arch in the rock and beyond it he could see a steep flight of wooden steps. His heart was pounding with excitement. There was no doubt now that they had discovered the smugglers’ secret. “We’ve found it,” he whispered to Joe. “We’ve found the passage. This must be directly underneath the house on the cliff.” “We’ll have to go quietly.”