the three of them decided it was not safe to go back to the open area tonight. after waiting a while longer still in the dark to see that their attacker was not coming, they searched the gloom around for a place to spend the rest of the night.

randy found an opening in the dense underbrush ahead of them. jill and ted followed him and his flashlight beam along the trail. suddenly they saw him stop dead in his tracks. ted walked abreast of him.

“what do you see?” ted asked.

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randy did not reply but instead shot his light ahead into the darkness. ted saw before them a huge cave entrance.

“gosh, do you suppose that’s the den of some wild animal?” ted asked.

“i don’t know,” randy answered in a quivery voice. “it seems like a good place to stay if it isn’t.”

jill had joined them by now. she too had taken some of the load of the spare oxygen cartridges.

“are we going into that spooky place?” jill asked.

“we can go up to it carefully and shine our light in,” ted said. “but we’d better be ready to run if something comes charging out! i wish i had that gun now!”

jill hung back as randy and ted moved stealthily forward toward the black cavern entrance. randy had his light shining directly into it all the time they were moving. when they were at the threshold of the cave, they got a good view of the interior.

“it’s not deep at all!” ted said. “it just goes back a little way.”

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“it looks deserted too,” randy added. “seems safe to me. what do you think, ted?”

“let’s go inside and see if there’s anything lying around,” ted suggested. “if it’s a den, there ought to be bones and things.”

cautiously they entered the cavern. its ceiling reached high over their heads and the opening was festooned with trailing vines and creepers. even the jungle growth seemed to have taken over, weeds and thick grass choking the floor. boulders of all sizes were scattered around.

“it looks like it hasn’t been used for years and years,” ted commented.

they flashed the light over the whole interior, but there was no sign of recent use. there was one other exit—a narrow passage at the rear.

“if we close up that rear opening with a big stone, it ought to be safe for us to stay here,” randy said.

ted agreed with him. they called jill, and the three shoved a large red boulder in front of the narrow passage. they divided watches again, but before relaxing for the night, they replaced their air cartridges with new ones.

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randy took first watch this time. ted was very tired from their exhausting race and had trouble falling asleep, but the next thing he knew, randy was shaking him to change watch.

the rest of the night passed without further disturbance. the boys got softhearted about calling on jill for her turn, and rather than wake her, they stood her duty. another change of air cylinders had to be made before morning. ted was able to change jill’s while she slept.

the orange glow of dawn was a welcome sight to the children. things did not seem half so grim in the dawn as they had the night before. the sun’s feeble rays shone directly into the cave mouth. the boulder covering the rear opening was still in place.

ted caught randy’s eyes staring thoughtfully at the boulder. he wondered if randy was thinking the same thing that he was: what was on the other side of that mysterious opening?

“hadn’t we better be getting back to the open place?” jill asked, as they were putting on fresh air tanks again.

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“the search party won’t be coming until a few hours yet,” randy said. “besides, it’s not very far.”

ted knew then that randy, too, was curious about the opening. he was stalling their return.

ted then came right out with it. “i’d sure like to know what’s on the other side of that rock.”

“why don’t we go and see?” randy said eagerly.

“we could go just a little way,” ted added, glancing at jill, whose face showed doubt. “just a few feet even.”

jill gave in grudgingly, but she got the boys to promise that they wouldn’t go far. “don’t forget, we’ve got some food back there,” she reminded them, “and i’m getting hungry.”

they left the air cartridges in the cave and walked through the enticing opening, ted in the lead. he flicked on his flashlight, for it was pitch dark. ahead of him he saw a narrow passageway. slowly he moved along it, randy and jill right behind him.

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they felt themselves tumbling downward.

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they were completely unprepared for the shock that next came to them. suddenly the ground dropped away under their feet, and they felt themselves tumbling downward!

all three of them cried out in terror as they fell. finally ted felt his body striking a cushioned surface. then he was rolling down an incline of the same soft material. down, down, head over heels he went—deeper and deeper into the core of the red planet, it seemed.

at last his body stopped turning. something crashed into him from behind. then he heard heavy breathing and gasping and he knew that it was either randy or jill who had collided with him.

“jill? randy?” he asked in a shuddery voice, still dazed by their rough experience.

“yes,” randy’s voice came weakly.

“jill!” ted cried. “where are you?”

“here i am,” she answered, from a few feet away. “what happened to us?”

“i don’t know,” her brother answered dully. he felt around for broken bones, but he appeared to be uninjured.

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“are you two all right?” he asked jill and randy.

they said they thought so. by now ted could see their forms very faintly. there was light coming from somewhere. their next task was to try to find a way out of this dismal place.

“i knew we should have gone back!” jill complained bitterly. “now we probably never will!”

“i’m sorry, sis,” ted said lamely. “you were right. i’m sure glad we changed our air tanks before we left!”

“let’s start looking for a way to the top,” randy said. “the search party will never find us down here.”

they discovered that the flashlight had been smashed in the fall. they would have to depend now on catlike vision to show them the way. as nearly as ted could make out, they were still in a corridor. it stretched mysteriously ahead of them, turning a bend about fifty feet away.

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“that seems to be the only way we can go,” ted said, looking forward. “we certainly can’t climb back up the way we came down.” he looked behind at the steep, rugged incline they had so unexpectedly tumbled down. the slope was covered with a matting of lichens or moss that had broken their fall.

they walked along the corridor. finally the light at the far end began to get brighter.

“it looks like daylight ahead!” jill said hopefully.

they increased their pace in the hope of finding a way leading back to the surface of the ground. they made a final turn in the winding underground aisle. then the corridor abruptly blossomed into a mammoth open area, still underground.

the sight that faced them quickened their heartbeats and made their mouths sag open in amazement. before them stood a towering iron gate, through which they could see evidence of one-time human habitation!

“what in the world have we found?” ted exclaimed.

“it must be a city!” randy burst out. “it is! we’ve found an underground martian city!”