"To the cutter!" Campbell cried. "That watch of mine was filled with the most concentrated high-explosive known, and it's blown up the tunnels. Now it's touched off more collapses and all these caverns and passages will fall in on us at any moment!"
The awful rumbling and crashing of collapsing rock masses was deafening in their ears as they lurched toward the cutter. Great chunks of rock were falling from the cavern roof into the water.
Sturt, white-faced but asking no questions, had the motor of the cutter running, and helped them pull the unconscious girl aboard.
"Out of the tunnel at once!" Campbell ordered. "Full speed!"
They roared down the water-tunnel at crazy velocity, the searchlight beam stabbing ahead. The tide had reached flood and turned, increasing the speed with which they dashed through the tunnel.
Masses of rock fell with loud splashes behind them, and all around them was still the ominous grinding of mighty weights of rock. The walls of the tunnel quivered repeatedly.
Sturt suddenly reversed the propellers, but in spite of his action the cutter smashed a moment later into a solid rock wall. It was a mass of rock forming an unbroken barrier across the water-tunnel, extending beneath the surface of the water.
"We're trapped!" cried Sturt. "A mass of the rock has settled here and blocked the tunnel."
"It can't be completely blocked!" Campbell exclaimed. "See, the tide still runs out beneath it. Our one chance is to swim out under the blocking mass of rock, before the whole cliff gives way!"