“There was a lake here up to a few years ago,” explained Jim.  “Bottom fell out and the water fell in.  Ain’t no bottom to it now at all”

“Then—–­then the water must have leaked out on the other side of the world,” stammered Chunky, his eyes big with wonder.

“I reckon it must have soused a heathen Chinee,” answered Nance, with a grin.

“Pity it didn’t fall out the other way and souse a few guides, eh?” questioned the fat boy, with a good-natured grimace at which Nance laughed inwardly, his shaking whiskers being the only evidence of any emotion whatever.

“Up there is Walnut Canyon,” explained Jim.  “Cliff dwellers lived up there some time ago.”

“Yes, we met some of them down south,” nodded Chunky.

“You mean we saw where they once lived long, long ago,” corrected Professor Zepplin.

“Yes, we saw where they lived,” agreed Stacy.

The way led on through a forest of pines, the trail underfoot being of lava, as hard and smooth as a road could be.  They were gradually drawing nearer to Sunset Mountain.  After a time they turned off to the right, heading straight for the mountain.

Tad rode back to the Professor to find out where they were going.

“I thought you boys might like to explore the mountain.  You will find some things there well worth scientific consideration.”

“Yes, sir; that will be fine.”

“You know the mountain was once a great volcano.”

“How long ago?” interrupted Stacy.

“A few million years or so.”

“Mr. Nance must have been a boy in short trousers then,” returned Stacy quizzically.  The guide’s whiskers bristled and stood out straight.

The road by this time had lost its hardness.  The ponies’ hoofs sank deep into the cinders, making progress slow for the party.  They managed to get to the base of the mountain, but the mustangs were pretty well fagged.  The animals were turned out for the night after having been hobbled so that they could not stray far away.

“Now each of you will have to carry a pack,” announced the guide.  “I will tell you what to take.”

“Why, where are we going?” asked Tad.

“We are going to spend the night in the crater of the extinct volcano,” said the Professor.  “Will not that be a strange experience?”

“Hurrah for the crater!” shouted the boys.

“Speaking of volcanoes, I wish you wouldn’t open your mouth so wide, Ned.  It makes me dizzy.  I’m afraid I’ll fall in,” growled Chunky.

CHAPTER IV

A NIGHT IN THE CRATER

“What, climb that mountain?” demanded Stacy.

“Surely.  You are not afraid of a mountain, are you?” demanded Tad.

“I’m not afraid of—–­of anything, but I’m delicate, I tell yau.”

“Just the same, you’ll pack about fifty pounds up the side of that hill,” jeered Ned Rector.