Walt, the practical, propounded the query, as they stood there, half-stunned by the rapidity with which unheard-of events had happened within the last half-hour. "Why, I--upon my word, I don't know," laughed Jack, brought up with a round turn by the hard-headed Walt. "I do," rejoined Walt. "What then?" "Escape to the open air." "You mean it?" Somehow, in his excitement, Jack had not gone as far as this daring suggestion. And yet it was, after all, the only thing to do. But suddenly another thought occurred to the boy. "The professor and Coyote Pete, how can we leave them?" "Well, we can't do them any good by remaining buried here, that's certain," replied Walt, in his sensible way. Jack and Ralph nodded agreement. "On the other hand, if this river really leads out into Mexico, we can take the subway to freedom and then, when we emerge, find out the best thing to do. Maybe we can fall in with some government troops or authorities of some kind." "But suppose the insurrectos are in power wherever this river comes out?" The question came from Ralph. "We'll have to take chances on that, I suppose." "Hark!" came suddenly from Jack. Far back somewhere in the tunnels they had threaded they could hear loud shouts and cries. The sound of the pursuit boomed out even above the noise of the waterfall. "They're after us!" exclaimed Jack. "Shall we take the boat?" Walt's usually calm voice shook a little as he asked the question. "It's our only chance. Come on, in with you, Ralph." Ralph hesitated no longer, but jumped into the little contrivance. A sort of oar lay in the bottom. He thrust it over the side. "The water's only about three feet deep," he announced. "So much the less chance of our being drowned," rejoined Jack. The lad had his knife out--a heavy-bladed hunting weapon. As soon as all was ready he would cut the ropes and set the boat free on the turbulent current. "All right!" cried Walt, as he clambered in and took his place by Ralph. Jack gave a hasty look around, and the next instant made a flying leap into the little craft. So fast had Black Ramon and his followers taken up the trail after they had discovered that the boys had found the secret of the hollow altar, that they were already entering the chamber. Ramon was in the lead. The glare of the lamp fell full on his parchment-like features, as with a roar of recognition, he sighted the boys. Ping! Something whizzed past Jack's ear, and, chipping the rock above, showered the occupants of the boat with fragments. The sharp report of the Mexican's revolver filled the place. With a quick movement, Jack slashed the rope nearest him. If he had not been in such a hurry, he would have seen that the other should have been severed first. As it was, he had cut the one that held the boat's bow to the stream. Instantly the flat-bottomed craft swung dizzily around, and still held by her stern mooring, dashed against the bank.