"Do you think you'd better?" asked Valerie.

"What else can we do?" put in Martia. "We can't just sit down here and form a colony of our own!"

Uncle Andy looked at the two women and their faces colored. "You asked for it!" he said, abruptly, and unscrewed the cap.

Beneath the cap were two tiny light bulbs embedded in a small panel, in addition to a red button. One of the lights glowed red.

"Well! Civilization at last! Shall I press the button?"

"I think Pee Bee may be right," said Henry. "They probably all went down under the ground and this is the control operating the hidden opening."

Uncle Andy looked up at him. "But if we go rushing in we're liable to end up captives too...."

In that moment, however, the decision was made for them. They discovered that the cairn marked the exact center of an area that was about fifty feet in diameter. This area suddenly sank downward.

"Run!" shouted Uncle Andy, springing to his feet.

But it was too late.