Could the light of day have penetrated to that mysterious and ancient underground city of gold our friends might have had some idea of its magnificence. As it was they could only view small parts of it at a time by the illumination of their electric torches. But even with them they saw that it was a most wonderful place.

"I don't believe there's another city like it in all the world," spoke Tom in awed accents, "there never was, and never will be again. Those Aztecs must have brought all their treasures of gold here."

"Bless my cake box! that's so," agreed Mr. Damon.

"Let's take a look around," advised Ned, "and then we can decide on what will be best to take away."

"It won't take me long t' make up mah mind," spoke Eradicate. "I's goin' t' take all dem images I kin find."

"I was going to say we'd have plenty of time to look about and pick what we wanted," said Tom, "but I think perhaps we'd better hurry."

"Why?" asked Mr. Damon.

"There's no telling when Delazes and his gang may find this place, and even the Fogers may be nearer than we think. But I believe our best plan would be this: To take some gold now, and several of the statues, go back to our balloon, and make some kind of big lamps, so we can light this place up. Then, too, I think we'd better move the balloon into the old temple. It will be safer there. Then we can come back here, pack up as much gold as we can carry, and be off. I don't like to think of being underground when Delazes and the Fogers are on the surface. It might not be altogether safe for us."

"Bless my insurance policy!" cried the odd man. "Now you're giving me the cold shivers, Tom. But I believe you're right. We must look ahead a bit."

With all their electric flash lamps turned on, the four advanced farther into the underground city of gold. As they went on they saw the precious yellow metal on every side of them. It was used lavishly, showing that to the ancients it was as common as iron or steel is to-day. But they did not use the gold merely as common material in the construction of buildings or objects of use. Instead, the gold seemed to be brought into play to beautify the city. An artistic scheme was carried out, and while it was true that in many buildings common objects were made of gold, yet each one was beautiful in itself.

"What a wonderful place this must have been when it was lighted up," spoke Tom.

"Do you think it was ever lighted up?" asked his chum.

"It must have been," declared the young inventor. "My idea is that this city was the home of the priests of the temple, and their friends. I don't believe the common people ever came here. Perhaps the officers of the army, the rulers and the royal family were admitted, but not the ordinary people. That's why it's so far underground, and so well guarded by the river."

"Probably the priests and others collected so much gold they didn't know what to do with it, and built this city to use it up, and, at the same time have a safe place to store it. And they must have had some means of lighting the place, for they couldn't go about in darkness--they couldn't have seen the gold if they did. Yes, this must have been wonderfully beautiful then. The priests probably came here to study, or perhaps to carry out some of their rites. Of course it's only guesswork, but it seems true to me."

"I believe you're right, Tom," said Mr. Damon.

As our friends walked about they saw that the city, while smaller than they had at first supposed, was laid out with regular streets. Each one was straight, and at certain places in the stone pavement plates of gold were set, so that literally the streets were paved with gold. There were houses or buildings on each side of the streets, and most of these were open at the doors or windows, for there was no need of heat in that buried city.

All about were the golden images such as they had seen in the Mexican's house, and like the one in far off Africa. Some of the images were almost life size, and others were only an inch or two inches in height. Not a house but had half a dozen or more in various places, and there were also the images on golden pedestals about the streets.

"This must have been their chief god, or else a representation of some great personage to whom they paid the highest honor," said Mr. Damon. "Perhaps he was the reigning king or ruler, and he, himself, might have ordered the images made out of vanity, like some men of to-day."

The boys agreed that this was a natural theory. As for Eradicate he was busy collecting numbers of the small golden statues, and stuffing them in his pockets.

"Why don't you take bigger ones, and not so many of them?" asked Tom.

"'Case as how I doan't want all mah eggs in one basket," replied the colored man. "I kin carry mo' ob de little fellers," and he persisted in this plan.

They found in some of the houses utensils of solid gold, but there appeared to be no way of cooking food, and that was probably done outside, or in the great temple. In many houses were articles evidently used in the sacrificial rites or in worship of strange gods. They did not stay to half examine the wonderful city of gold, for it would have taken several days. But on Tom's advice, they took up a considerable quantity of the precious metal in the most convenient form to carry, including a number of the statues and art objects and started back along the tunnel.

"We'll rig up some sort of lamps," Tom explained, "and come back to make a thorough examination of this place. I think the scientific men and historians will be glad to know about this city, and I'm going to make some notes about it."

They soon came again to the place of the underground river and found no water there. Ned wanted to turn the stream back into the channel again, but Tom said they might not be able to work the ancient mechanism, so they left the black knob as it was, and hurried on. They decided that the knob must have worked some counter-balance, or great weight that let down a gate and cut off the river from one channel, to turn it into another.

When they emerged at the top of the steps, and came out at the opening which had been revealed by the rolling back of the great altar, they saw there that counter weights, delicately balanced, had moved the big stone.

"We might close that opening," said Tom, "and then if any one should come along and surprise us, they wouldn't know how to get to the underground city." This was done, the altar rolling back over the staircase.

"Now to get the balloon in the temple, make the lamps, and go back," suggested Tom, and, storing the gold they had secured in a safe place in the temple, they went back to move the airship.

This was an easy matter, and soon they had floated the big gas bag and car in through one of the immense doorways and so into the great middle part of the temple where the big stone altar was located.

"Now we're prepared for emergencies," remarked Tom, as he looked up at the yawning hole in the dome-like roof. "If worst comes to worst, and we have to run, we can float right up here, out of the temple, and skip."

"Do you think anything is going to happen?" asked Mr. Damon anxiously.

"You never can tell," replied Tom. "Now to make some lamps. I think I'll use gas, as I've got plenty of the chemicals."

It took two days to construct them, and Tom ingeniously made them out of some empty tins that had contained meat and other foods. The tins were converted into tanks, and from each one rose a short piece of pipe that ended in a gas tip. On board the dirigible were plenty of tools and materials. Into the cans were put certain chemicals that generated a gas which, when lighted, gave a brilliant glow, almost like calcium carbide.

"Now, I guess we can see to make our way about," remarked Tom, on the morning of the third day, when they prepared to go back to the city of gold. "And we'll take plenty of lunch along, for we may stay until nearly night."

It did not take them long to roll back the altar, descend into the tunnel, and reach the underground city. The river channel was now dry, even the small pools of water in the depressions having evaporated.

The gas torches worked to perfection, and revealed the beauties and wonders of the city of gold to the astonished gaze of our friends. It was even richer in the precious metal than they had at first supposed.

"Before we do any exploring, I think we'd better take some more gold back to the balloon," suggested Tom, "and I think I'll just move the balloon itself more out of sight, so that if any persons come along, and look into the temple, they won't see our airship without looking for it."

This was done, and a considerable quantity of the precious metal, including a number of the larger-sized statues, were stored in the balloon car.

"We can't take much more," Tom warned his friends, "or we'll be over-weighted."

"We've got enough now, to make us all rich," said Ned, contentedly.

"I want moah," spoke Eradicate with a grin.

They went back to the underground city and began to explore it with a view of taking back to civilization some word of its wonders and beauties.

"Didn't Mr. Illingway, in his letters, say something about an immense golden statue here?" asked Ned, when they had almost completed a circuit of the underground place.

"So he did!" exclaimed Tom. "I'd almost forgotten. It must be somewhere in the centre of this place I should think. Let's have a hunt for it. We can't take it with us, but maybe we could get part of an arm or a leg to keep as a relic. Come on."

It was easy to reach the centre of the underground city, for it was laid out on a regular plan. In a short time they were in sight of the central plaza and, even before they reached it the glare of their gas lamps showed them something glittering golden yellow. It was on a tall, golden pedestal.

"There it is!" cried Ned.

"Yes, there's the big golden image all right," agreed Tom, hurrying forward, and a moment later they stood before a most wonderful statue.