But just then, in the moment of their elation, a terrifying thing happened. As if aroused by their voices, the still surface a few yards from shore boiled up, and was lashed to foam by the strokes of a gigantic tail. "Run!" yelled Grom; and they all dashed forward, there being no chance to go back. In the same instant, an appalling head--like that of a thrice magnified and distorted crocodile, with vast, round, painted eyes--was upthrust from the water and came rushing after them at a pace which sent up a curving wave before it. Quick as thought, Grom drew his bow and shot at the appalling head. The arrow drove straight into the gaping throat, eliciting a thunderous bellow of rage, but producing no other effect. Then Grom sprang after his fleeing companions, and raced for his life toward the cave mouth. The cave might be nothing more than a death-trap for them all; but it seemed to offer the one possibility of escape. As they dashed into the cave the awful, gaping head was close behind them. They had a flashing glimpse, through the gloom, of high-arched distance melting into blackness, of a strip of black water along the right, and to the left a gentle ascent of smooth white sand, whose end was out of sight. Up this slope they raced, with the clashing of monstrous fangs close behind them. But they had not gone a dozen strides when the slope quivered, and heaved upwards shudderingly beneath them; and they all fell forward flat upon their faces. From all but Grom there went up a shriek so piercing that in their own ears it disguised the stupendous rending roar which at that moment seemed to stun the air. The mighty arch of the cave mouth had slipped and crashed down, completely jamming the entrance, and opening up a gash of blue heaven above their heads. To Grom's unshaken wits, it was clear on the instant what had happened. He staggered to his feet and looked back through a rain of falling rock-splinters. He had a vision of their colossal pursuer, its jaws stretched to their utmost width, the vast globes of its eyes protruding from their armored sockets, its ponderous, bowed fore-legs pawing the air aimlessly in the final convulsion. The falling rock-mass had caught it on the middle of the back, crushing its mighty frame like an eggshell. For a second or two, Grom stood there rigid, staring, his gnarled fingers clenched upon his weapons. Then a second earthquake tremor beneath his feet warned him. With an unerring instinct, he sprang on up the slope after his companions, who had fled as soon as they could pick themselves up. And in the next moment the rock above his head, fissured deep by the rains, slipped again. With a growling screech, as if torn from the bowels of the mountain, it settled slowly down, and sealed the mouth of the cave to utter blackness.