Gathered beyond the mouth of the tunnel, far enough away so that the wind of the great blast would not bowl them over like ten pins, stood Tom Swift and his friends. In his hand Tom held the battery box, the setting of the switch in which would complete the electrical circuit and set off the hundreds of pounds of explosive buried deep in the hard rock.

"Are all the men out?" asked the young inventor of Tim Sullivan, who had charge of this important matter. Tim was in sole charge as foreman now, having picked up enough of the Indian language to get along without an interpreter.

"All out, sor," Tim responded. "Yez kin fire whin ready, Mr. Swift."

It was a portentous moment. No wonder Tom Swift hesitated. In a sense he and his friends, the contractors, had staked their all on a single throw. If this blast failed it was not likely that another would succeed, even if ther should be time to prepare one.

The time limit had almost expired, and there was still a half mile of hard rock between the last heading and the farther end of the big tunnel. If the blast succeeded enough rock might be brought down to enable the work to go on, by using a night and day shift of men. Then, too, there was the chance that the hard strata of rock would come to an end and softer stone, or easily-dug dirt, be encountered.

"Well, we may as well have it over with," said Tom in a low voice. Every one was very quiet--tensely quiet.

The young inventor looked up to see Professor Bumper observing him.

"Why, Professor!" Tom exclaimed, "I thought you had gone off to the mountains again, looking for the lost city."

"I am going, Tom, very soon. I thought I would stop and see the effect of your big blast. This is my last trip. If I do not find the hidden city of Pelone this time, I am going to give up."

"Give up!" cried Mr. Damon. "Bless my fountain pen!"

"Oh, not altogether," went on the bald-headed scientist. "I mean I will give up searching in this part of Peru, and go elsewhere. But I will never completely give up the search, for I am sure the hidden city exists somewhere under these mountains," and he looked off toward the snow-covered peaks of the Andes.

Tom looked at the battery box. He drew a long breath, and said:

"Here she goes!"

There was a contraction of his hand as he pressed the switch over, and then, for perhaps a half second, nothing happened. Just for an instant Tom feared something had gone wrong that the electric current had failed, or that the wires had become disconnected--perhaps through some action of the plotting rivals.

And then, gently at first, but with increasing intensity, the solid ground on which they were all standing seemed to rock and sway, to heave itself up, and then sink down.

"Bless my--" began Mr. Damon, but he got no further, for a mighty gust of wind swept out of the tunnel, and blew off his hat. That gust was but a gentle breeze, though, compared to what followed. For there came such a rush of air that it almost blew over those standing near the opening of the great shaft driven under the mountain. There was a roar as of Niagara, a howling as in the Cave of the Winds, and they all bent to the blast.

Then followed a dull, rumbling roar, not as loud as might have been expected, but awful in its intensity. Deep down under the very foundations of the earth it seemed to rumble.

"Run! Run back!" cried Tom Swift. "There's a back-draft and the powder gas is poisonous. Stoop down and run back!"

They understood what he meant. The vapor from the powder was deadly if breathed in a confined space. Even in the open it gave one a terrible headache. And Tom could see floating out of the tunnel the first wisps of smoke from the fired explosive. It was lighter than air, and would rise. Hence the necessity, as in a smoke-filled room, of keeping low down where the air is purer.

They all rushed back, stooping low. Mr. Damon stumbled and fell, but Koku picked him up and, tucking him under one arm, as he might have done a child, the giant followed Tom to a place of safety.

"Well, Tom, it went off all right," said Mr. Job Titus, as they stood among the shacks of the workmen and watched the smoke pouring out of the tunnel mouth.

"Yes, it went off. But did it do the work? That's what we've got to find out."

They waited impatiently for the deadly vapor to clear out of the tunnel. It was more than an hour before they dared venture in, and then it was with smarting eyes and puckered throats. But the atmosphere was quickly clearing.

"Switch on the lights," cried Tom to Tim, for the illuminating current had been cut off when the blast was fired. "Let's see what we've brought down."

Following the eager young inventor came the contractors, some of the white workers, Mr. Damon and Professor Bumper. The little scientist said he would like to see the effect of the big blast.

Along they stumbled over pieces of rock, large and small.

"Some force to it," observed Job Titus, as he observed pieces of rock close to the mouth of the tunnel. "If it only exerted the force the other way, against the face of the rock, as well as back this way, we'll be all right."

"The greater force was in the opposite direction," Tom said.

A big search-light had been got ready to flash on the place where the blast had been set off. This was to enable them to see how much rock had been torn away. And, as they reached the place where the flint-like wall had been, they saw a strange sight.

"Bless my strawberry short-cake!" gasped Mr. Damon. "What a hole!"

"It is a bole," admitted Tom, in a low voice. "A bigger hole than I dared hope for."

For a great cave, seemingly, had been blown in the face of the rock wall that had hindered the progress of the tunnel. A great black void confronted them.

"Shift the light over this way," called Tom to Walter Titus, who was operating it. "I can't see anything."

The great beam of light flashed into the void, and then a murmur of awe came from every throat.

For there, revealed in the powerful electrical rays, was what seemed to be a long tunnel, high and wide, as smooth as a paved street. And on either side of it were what appeared to be buildings, some low, others taller. And, branching off from the main tunnel, or street, were other passages, also lined with buildings, some of which had crumbled to ruins.

"Bless my dictionary!" cried Mr. Damon. "What is it?"

Professor Bumper had crawled forward over the mass of broken rock. He gazed as if fascinated at what the searchlight showed, and then he cried:

"I have found it! I have found it! The hidden city of Pelone!"