From a case which held his special equipment, Doc took a pad and pencil. He wrote a number of several figures. Eyes shut, he extracted the square and cube root in his head, carrying the figures to many decimal places.

Out of the case came a device which made sound waves of all tones, some of a wave length so short or so long as to be inaudible to the normal ear. Years of straining to detect these waves had enabled Doc to make his ears sensitive enough to hear many sounds inaudible to ordinary people.

With his eyes closed, Doc rapidly catalogued by the sense of smell several score of different odors, all very vague, each contained in a small vial racked in the case.

There were other exercises, far more intricate. Ham shook his head wonderingly. He knew that five minutes at the clip Doc was doing the routine would be more than he, himself, could stand. And Ham was husky enough to give most professional boxers a drubbing.

From the cradle, Doc had done these exercises each day. They accounted for his astounding physique, his ability to concentrate, and his superkeen senses.

"What's on your mind?" Doc asked suddenly. His routine was over!

Ham plucked a newspaper out of a pocket.

"What do you think of this?" He handed Doc the paper, indicating an item, It read:

WANT TO BUY A POLAR

SUBMARINE EXPEDITION?

There is one for sale. Captain Chauncey McCluskey

announced this morning that he is hunting a purchaser for a

share of the projected trip of the submarine Helldiver under

the polar ice.

Captain McCluskey has the submarine, fully equipped and

ready to go. But it seems he has run out of money.

There was more of it, written up in typical tabloid style. But it told nothing more of importance — except that the submarine Helldiver was tied up at a local pier, and Captain Chauncey McCluskey could be found aboard.

"Who is Captain McCluskey?" Ham inquired.

Doc shook his head slowly. "Search me! I never heard of the man before. Nor have I heard of any other projected submarine trip under the pole."

"This sub may be just what we need," Ham declared. "But there's one point which has me guessing. It's darn queer the thing should pop up at just the time we're interested."

Doc smiled slightly. "It won't hurt to look into it, anyway."

The regular elevator — not the super-speed one — lowered them to the street level.

They took the first taxi which rolled up.

Doc gave their driver the address of the pier to which was moored the polar submarine, Helldiver.

Office workers were going to their daily tasks. The walks were crowded. Each subway kiosk vomited humanity like an opened anthill. The cab rolled down into a cheaper district, where merchants were setting a part of their wares out on the walks.

Ham toyed with his sword cane, and wondered what kind of a tub the Helldiver would be.

Suddenly he snapped rigid as an icicle.

In to the cab had permeated the low, mellow sound which was part of Doc. Weird, exotic, the note trilled up and down the musical scale. Looking directly at Doc's strong lips, Ham could not tell the sound was coming from them, such a quality of ventriloquism did the trilling note have. Indeed, Doc himself probably did not quite realize he was making it..

The sound could have but one meaning now.

Danger!

"What is it?" Ham demanded.

"Listen!" Doc told him abruptly.

Silence lasted about a minute. Then Ham's high, intelligent forehead acquired a dubious pucker.

"I hear a clicking noise at intervals, I think," he said. "Sounds like somebody shaking a couple of dice!"

"Remember the clicking noise Victor Vail mentioned having heard often during the past years?"

Ham never got to say whether he recollected or not.

Their driver suddenly flicked several small objects back into the tonneau. He was careful to keep his face from being seen.

The objects he flung were the grape-like balls of anesthetic Doc had used to overpower Ben O'Gard's hired gangsters. No doubt these had come from the scene of that affair, since Doc had neglected to retrieve such of them as had not been broken.

The globules shattered.

Doc and Ham were caught. With hardly a quiver, they tumbled over unconscious on the cushions.

They had not glimpsed the countenance of their driver.

HAM sat up. He groaned loudly.

"If you're complaining about the darkness," came Doc's steady, capable voice, "that's why you can't see anything. And as for where we are — we seem to be inside a steel vault."

"What a dream I had waking up!" Ham muttered.

"The anesthetic sometimes has that effect. I judge we've been unconscious nearly two hours. One shot of the anesthetic lays a man out for about that long."

Ham suddenly clutched at various parts of his person. His hands made loud slaps on his bare hide.

"Hey!" he yelled. "I've only my underclothes!"

"So have I," Doc told him. "They took our clothing. They even combed our hair, from the way mine feels. And they swept the interior of the vault clean. There are no shelves, or anything else — except a candle and three matches which they kindly left us."

"Light the candle," Ham suggested. "This place is blacker than the inside of an African savage!"

"No, Ham," Doc replied. "They left the candle, hoping we'd light it."

"Huh?" Ham was puzzled.

"A flame will exhaust the oxygen in this place very quickly, and hasten our death by suffocation."

''You mean the vault is airtight? "

"Yes. And soundproof, too."

Ham now listened. He realized he could not hear a sound but the booming of his own heart. It was so quiet he could almost hear the blood gurgle through his arteries. He shivered. A heavy lead weight seemed to climb on his chest.

"The air in here must be pretty foul already," be muttered.

"Very," Doc agreed. "I have been thinking, Ham. You recall that some months ago a large chain of New York banks went out of business. Probably we are in the vault of one of those banks."

"Ugh!" Ham shuddered. "Can't you think of something cheerful?"

Doc Savage's low laugh vibrated through the awful steel cubicle. He rarely laughed.

"How's this for something cheerful?" he inquired. "As a matter of fact, I've only been waiting for you to regain consciousness before walking out of this place."

HAM EMITTED a howl of delight that was almost a sob. He sprang erect. They were two semi-naked men inclosed in thick walls of hard steel. Their voices could not penetrate outside, just as no sounds could get in. The situation seemed hopeless.

But Doc Savage had a way! He never joked about matters as serious as this.

"How do we do it?" Ham demanded.

"Our captors probably looked in our mouths," Doc explained. "But they forgot to count my teeth. They didn't notice that in my upper jaw there is an extra wisdom tooth on each side. They're false, and they hold two chemical compounds of my own concoction. When combined, these form one of the most powerful explosives."

Doc now went to work on the vault door. He operated in darkness, guided only by his sensitive finger tips.

"Kind of them to leave us the candle," Doc said.

He used the candle wax to chink his explosive in the joint of the vault door, near the lock.

"Get in a corner!" he directed Ham.