"I believe we've tumbled upon the approach to a submarine tunnel to Sandinsel," whispered Detroit.
"Much too wide for that," replied the Sub, "unless, of course, there is a subsidiary tunnel. But, look, we are getting to the end. There's rock ahead of us."
The incline terminated in a large circular basin roughly two hundred yards in diameter. Opening out of it were three lofty artificial caves, hewn out of the sandstone. This much Hamerton was able to observe in a momentary burst of starlight through a rift in the swiftly-moving clouds.
"Come on," he said. "We're in luck's way. There's no one about. Follow the cliff; I'm curious to see what is in these caves."
The first cavern was apparently empty. The floor was paved with slabs of cement, the walls were of glared brick. Close to the mouth was a little hut, the door of which was locked; but judging by the number of insulated wires running from it the Sub concluded it was the operating station for lighting the vast recess.
The second cave, its entrance being almost at an angle of ninety degrees with the first, was tenanted. Both men recognized in the snub-nosed object projecting far above their heads the bow of a military Zeppelin.
Hamerton could have danced with sheer delight. He had discovered what was supposed to be the shell-proof lair of three powerful airships. Deep in the bowels of the earth they were immune from hostile shell. A high-angled fire might result in a projectile dropping into the vast circular pit, but the possibility of the aircraft being damaged by fragments of shells was guarded against by the provision of massive steel doors sliding on rollers.
Evidently the Zeppelins were being made ready for a flight, for the doors were wide open. In the third cavern lights burned at the farthermost end, and a party of officers and men were busily engaged in overhauling the after-car.
"Seen enough?" asked Detroit. "Let's quit."
"Suppose we must," replied Hamerton reluctantly. "It must be close on dawn, and we'll have to find a place to stow ourselves away."