Jounced against the rough, rock walls, bumped over shoal places, and at times whirled almost broadside on by the swift current, the queer, flat-bottomed boat containing our three young friends was hurried through the darkness. It was the maddest ride any of them had ever taken, and, as we know, they had been through some thrilling experiences since they had first stood on the railroad station platform at Maguez. Had they known it, they could have controlled the boat more or less with the rough oar--the one with which Ralph had sounded the depth of the river--but, of course, they were inexpert in the management of such a craft. They could do nothing but keep still and trust to luck to bring them safely out of their extraordinary predicament. After some ten minutes of this, the current seemed to slacken a little and the walls narrowed. Jack stretched out a hand and, to his astonishment, his fingers were swept along a rope stretched down the side of the tunnel. This solved a problem he had been revolving in his mind--namely, how did the Mexicans get their boat back after it had delivered its cargo of arms? The explanation was now a simple one. Evidently they hauled it back by the use of this rope. "It must have been hard work, though," thought Jack. Conversation was impossible in the confines of the tunnel which, in places, was a mere tube in the rocks; the roar of the water was almost deafening. It was so black, too, that they could not see one another's faces. Of real alarm Jack did not feel much, and for an excellent reason. It was apparent that the Mexicans had used this underground route across the border many times, and, if they could make the passage--terrifying as it seemed--in safety, there was every reason to suppose that the boys could make it with the same security. What worried Jack most about their situation proceeded from a far different cause. There was little reason to doubt that at the other end of the tunnel, wherever that might be, Black Ramon or his superiors, arming the insurrectionists, had guards posted to receive the smuggled guns. If no opportunity of escaping from the boat presented itself before they were hastened out of the exit of the tunnel, their situation would be just as bad as ever. Ramon would, of course, lose no time in following them up, either by a spare boat, which he might have had concealed in the vaulted chamber, or else on his fast, coal-black horse which he might ride across the rocky range, far above the subterranean stream.