In a few minutes a fire was blazing, lighting up the interior of the crater. The boys found themselves in a circular opening of almost terrifying roughness and something like a quarter of a mile across.

Here, in ages past, the forces of Nature had been at work with fearful earnestness. Weird shadows, mysterious shapes, somewhat resembling moving figures, were thrown by the flickering blaze of the camp fire.

While the boys were exploring the crater Dad was busy getting the supper ready, talking with Professor Zepplin as he worked.

The voices of the boys echoed from side to side of the crater, sounding strange and unreal. The call to supper put an end to their explorations.

They sat down with keen edges to their appet.i.tes. It was their first meal in the open on this journey. All were in high spirits.

"I think we should agree upon our work for the future," declared the Professor.

"Work?" exclaimed Chunky, opening wide his big eyes.

"Yes. It is not going to be all play during this trip."

"We are willing to do our share," answered Ned.

"Yes, of course we are," chorused Walt and Stacy, though there was no enthusiasm in the fat boy's tone.

"I am of the opinion that you boys should take turns in cooking the meals, say one boy to cook for an entire day, another to take the job on the following day."

"I'll cook my own," declared the guide. "No tenderfoot experiments in my chuck."

"They know how to cook, Mr. Nance," explained the Professor.

"All right; they may cook for you," said the guide, with a note of finality in his tone. He glanced up at the sky, held out his hand and shook his head. Tad observed the movement.

"What is it?" asked the boy.

"It's going to snow," said Dad.

Tad laughed, glancing at his companions.

"What, snow in June?" questioned Stacy.

"You must remember that you are a good many thousand feet up," the Professor informed him.

"Up? I thought I was down in a crater."

"You are both up and down," spoke up Tad.

"Yes, I'm usually up and down, first standing on my feet then on my head," retorted Stacy. "How are we going to sleep?"

"Same as usual. Pick out your beds, then roll up in your blankets,"

directed Dad. "You are used to it, eh?"

"Well," drawled Chunky, "I've slept in a good many different kinds of beds, but this is the first time I ever slept in a lava bed."

True to Dad's prophecy, the snow came within half an hour.

"Better turn in before the beds get too wet," advised Dad.

All hands turned in. Sleep did not come to the boys as readily as usual.

They had been sleeping in real beds too long. After a time the snow changed to rain in the warmth of the crater. Chunky got up disgustedly.

"I'm tired of sleeping in the bath tub," he declared. "Think I'll move into the hall bedroom."

Chuckles were heard from beneath other blankets, while Stacy, grumbling and growling, fussed about until he found a place that appeared to be to his liking.

"When you get through changing beds perhaps you will give us a chance to go to sleep," called the guide.

Stacy's voice died away to an indistinct murmur. Soon after that quiet settled over the dark hole in the mountain. The rain came down harder than ever, but by this time the Pony Rider Boys were asleep. They neither heard nor felt the water, though every one was drenched to the skin.

Toward morning Tad woke up with a start. He thought something had startled him. Just then an unearthly yell woke the echoes of the crater. Yell upon yell followed for the next few seconds, each yell seeming to be further away than the preceding one, and finally dying out altogether.

"It's Chunky!" shouted Tad, kicking himself free of his blankets and leaping up. "Some thing's happened to Chunky!"

CHAPTER V

TAD LENDS HELPING HAND

"What is it? What is it?" cried the other boys, getting free of their blankets and in the confusion rolling and kicking about in the cinders.

"What is it?" shouted the Professor, very much excited.

Ned, dragging his blanket after him, had started to run about, not knowing which way to turn nor what had occurred. In the meantime the guide and Tad had started in the direction from which the yells had seemed to come.

"It was this way," shouted Tad.

Ned headed them off running toward the west edge of the crater. All at once a new note sounded. With an unearthly howl Ned Rector disappeared.

They heard his voice growing fainter, too, just as Stacy's had done.

"They've fallen in!" cried Tad.

"Everybody stand still!" commanded Dad.

Recognizing that he was right, the others obeyed, with the exception of Tad Butler, who crept cautiously forward, feeling his way with the toes of his boots, that he too might not share the fate of his two companions.

Dad, from somewhere about his person, produced a bundle of sticks which he lighted. He was prepared for just such an emergency. A flickering light pierced the deep shadows, just enough to show the party that two of their number had disappeared.

"There is the place," cried Tad. "It's a hole in the ground. They've fallen in."

"Chunky's always falling in," laughed Walter half hysterically.

With his rope in hand, Tad sprang forward.

"Light this way, please," called Butler. "h.e.l.lo, down there!" he cried, peering into the hole in the ground.