“Oh, they probably died—­from a plague maybe, or there may have been a war, or the people may have risen in revolt and killed them off,” suggested Tom grimly.

“But then there ought to be some remains—­some mummies or skeletons or something.”

“I guess every one left this underground city—­every soul.” suggested Mr. Damon, “and then they turned on the river and left it.  I shouldn’t be surprised but what we are the first persons to set foot here in thousands of years.”

“And we may stay here for a thousand years,” predicted Tom.

“Oh, good land a’ massy; doan’t say dat!” cried Eradicate.  “Why we’ll all be dead ob starvation in dat time.”

“Before then, I guess,” muttered Tom.  “I wonder if there’s any water in this hole?”

“We’ll need it—­soon,” remarked Ned, looking at the scanty supply they had brought in with them.  “Let’s have a hunt for it.”

“Let Rad do that, while we work on the stone gate,” proposed the young inventor.  “Rad, chase off and see if you can find some water.”

While the colored man was gone, Tom, Ned and Mr. Damon went back to the stone gate.  To attack it without tools, or some powerful blasting powder seemed useless, but their case was desperate and they knew they must do something.

“We’ll try chipping away the stone at the base,” suggested Tom.  “It isn’t a very hard rock, in fact it’s a sort of soft marble, or white sand stone, and we may be able to cut out a way under the slab door with our knifes.”

Fortunately they had knives with big, strong blades, and as Tom had said, the stone was comparatively soft.  But, after several hours’ work they only had a small depression under the stone door.

“At this rate it will take a month,” sighed Ned.

“Say!” cried Tom, “we’re foolish.  We should try to cut through the stone slab itself.  It can’t be so very thick.  And another thing.  I’m going to play the flames from the gas torches on the stone.  The fires will make it brittle and it will chip off easier.”

This was so, but even with that advantage they had only made a slight impression on the solid stone door after more than four hours of work, and Eradicate came back, with a hopeless look on his face, to report that he had been unable to find water.

“Then we’ve got to save every drop of what we’ve got,” declared Tom.  “Short rations for everybody.”

“And our lights, too,” added Mr. Damon.  “We must save them.”

“All out but one!” cried Tom quickly.  “If we’re careful we can make them gas torches last a week, and the electric flashes are good for several days yet.”

Then they laid out a plan of procedure, and divided the food into as small rations as would support life.  It was grim work, but it had to be done.  They found, with care, that they might live for four days on the food and water and then—­

Well—­no one liked to think about it.