“Sounds like thunder.”

The others listened.  Sure enough there was a dull rumble and roar audible.  It seemed off to the left, but they could see no clouds in the sky, nor any signs of a storm.

“Let’s take a walk over that way and see what it is,” Mr. Henderson suggested.

As they walked on the noise became louder, until in about half an hour it was like the sound from a blast furnace.

“What do you suppose it can be?” asked Mark.

“Perhaps some new freak of nature,” the professor replied.  “We seem to have a good many of them here.”

They were all on their guard now, for there was no telling into what danger they might run.  As they went up a little hill the noise became much louder.  The professor and Andy, who had taken the lead, kept a sharp lookout ahead, that they might not unexpectedly fall into some hidden stream or lake.  As they topped the hill they saw before them a deep valley, and in the midst of it was that which was causing the roaring sound.

From the centre of an immense mound of rock and earth there spouted up a great column of water, three hundred feet or more, as straight as a flag staff.  It was about ten feet in diameter, and at the top it broke into a rosette of sparkling liquid, which as the vari-colored lights played on it, resembled some wonderful flower.

“It’s a great geyser!” the professor exclaimed.  “We have come to a place like Yellowstone Park.  We must be very careful.  The crust may be very thin here, and let us down into some boiling spring.”

The others gathered around the professor, and, from a safe distance watched the ever rising and falling shaft of water.

It was not regular in motion.  Sometimes it would shoot up to a great distance, nearly a thousand feet, the professor estimated.  Again it would sink down, as the power sending it out lessened, until it was only a few hundred feet above the rounded top of the mound from which it spurted.  But it never fell below this.  All the while there was the constant roaring sound, as though the forces of nature below the surface were calling to be let out.

“I hope there are not many of those about,” Mr. Henderson remarked after a pause.  “If the ship should hit one during the night it would be all up with us.  We must keep a careful look-out.”

The spouting column had a fascination which held them to the spot for some time.  From the hill they had a good view of the surrounding country, but did not see any more geysers.

“Do you think it is hot water?” asked Mark.

“There is no vapor,” the professor answered, “but most of the geysers are produced by the action of steam in the interior of the earth.  However we’ll not take any chances by investigating.  I fear it would not be safe to go into that valley.”

“Look there!” cried Andy.  “I guess we’re better off here!” He pointed a little to the right of where the water spouted.  The others looked, and saw, coming from a hole in the ground, some shaggy black object.