it back down the tunnel with all his force.
"Quick—out of the tunnels now or we'll die right here!" he yelled.
They lunged forward, Campbell dragging both the girl and the exhausted Ennis, and emerged a moment later into the great water-cavern. It was now lit only by the searchlight of their waiting cutter.
As they emerged into the cavern, they were thrown flat on the rock ledge by a violent movement of it under them. An awful detonation and thunderous crashing of falling rock smote their ears.
Following that first tremendous crash, giant rumbling of collapsing rock shook the water-cavern.
"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!"
"But there's no telling how far the block may extend" Sturt cried.
Then as Campbell and Ennis stripped off their coats and shoes, he followed their example. The rumble of grinding rock around them was now continuous and nerve-shattering.
Campbell helped Ennis lower Ruth's unconscious form into the water.
"Keep your hand over her nose and mouth!" cried the inspector. "Come on, now!"
Sturt went first, his face pale in the searchlight beam as he dived under the rock mass. The tidal current carried him out of sight in a moment.
Then, holding the girl between them, and with Ennis' hand covering her mouth and nostrils, the other two dived. Down through the cold waters they shot, and then the swift current was carrying them forward like a mill-race, their bodies bumping and scraping against the rock mass overhead.
Ennis' lungs began to burn, his brain to reel, as they rushed on in the waters, still holding the girl tightly. They struck solid rock, a wall across their way. The current sucked them downward, to a small opening at the bottom. They wedged in it, struggled fiercely, then tore through it. They rose on the other side
