“Some one’s coming,” whispered Price.  “Get ready.”

We levelled our guns.  I was about to fire.

“Look out!  Don’t shoot!” warned a voice sharply.  It was Elaine.  Her keen eyes and quick perception had recognized Arnold, leading Woodward.  We lowered our guns.

“Did you see a man, masked, come out here?” cried Woodward.

“No—­he must have gone your way,” we called.

“No, he couldn’t.”

Arnold was eagerly questioning the captain as Elaine and I approached.  “Dropped into the water—­risked almost certain death,” he muttered, half turning and seeing us.

“I want to congratulate you on your nerve for going in there,” began Elaine, advancing toward the professor.

Apparently he neither heard nor saw us, for he turned as soon as he had finished with Price and went into the cave as though he were too busy to pay any attention to anything else.

Elaine looked up at me, in blank astonishment.

“What an impolite man,” she murmured, gazing at the figure all stooped over as it disappeared in the darkness of the tunnel.

CHAPTER XII

THE DEATH CLOUD

Off a lonely wharf in a deserted part of the coast some miles from the promontory which afforded Del Mar his secret submarine harbor, a ship was riding at anchor.

On the wharf a group of men, husky lascars, were straining their eyes at the mysterious craft.

“Here she comes,” muttered one of the men, “at last.”

From the ship a large yawl had put out.  As she approached the wharf it could be seen that she was loaded to the gunwales with cases and boxes.  She drew up close to the wharf and the men fell to unloading her, lifting up the boxes as though they were weighted with feathers instead of metal and explosives.

Down the shore, at the same time, behind a huge rock, crouched a rough looking tramp.  His interest in the yawl and its cargo was even keener than that of the lascars.

“Supplies,” he muttered, moving back cautiously and up the bluff.  “I wonder where they are taking them?”

Marcus Del Mar had chosen an old and ruined hotel not far from the shore as his storehouse and arsenal.  Already he was there, pacing up and down the rotted veranda which shook under his weight.

“Come, hurry up,” he called impatiently as the first of the men carrying a huge box on his back made his appearance up the hill.

One after another they trooped in and Del Mar led them to the hotel, unlocking the door.

Inside, the old hostelry was quite as ramshackle as outside.  What had once been the dining-room now held nothing but a long, rickety table and several chairs.

“Put them there,” ordered Del Mar, directing the disposal of the cases.  “Then you can begin work.  I shall be back soon.”

He went out and as he did so, two men seized guns from a corner near-by and followed him.  On the veranda he paused and turned to the men.