"As the centuries passed, the atmosphere continued to thin," Staunton went on, "so the Ancients took care to preserve what was left. Gral-Thala is in the fertile part of the Moon, and lies in a vast valley completely surrounded by a lofty mountain range. By means of the superior engineering knowledge of the Ancients, they built a lofty wall or barrier along the crest of the range so that its top is miles above the level of the valley floor. They then sucked all the air within the Great Barrier. Gral-Thala itself thus lies in a great pool of air surrounded by the ranges and the barrier. On the rest of the Moon, as here, air only remains in deep crevices and caverns like this." "But these caves were a great labor in themselves..." Ripon began. * * * * * "Originally these caverns were built as outposts of Gral-Thala, built here because of their nearness to valuable mineral deposits. People came out from the sunlit cities within the Great Barrier to put in a tour of duty in the caverns. Again life on the Moon had reached a pleasant equilibrium. And then came the great disaster! Some two centuries ago a group of several hundred outlaws fleeing from Earth came here in a big space-ship." "The _Mercury_!" Larry exclaimed. "Exactly. Those men and women who came from Earth were few in comparison to the population of the Moon, but they were cruel and ruthless and they had weapons of war. The peaceful Lunarians had at that time no weapons at all, for they had no need for them. Within a few months the invaders made themselves Lords of all Gral-Thala! That was when English, the language of the invaders, came to be spoken by everybody on the Moon as well as the softer tongue of the Lunarians themselves. A few of the hardier folk in Gral-Thala fled to these caverns as outlaws. The invaders made only half-hearted attempts to come after them, and with the passing of the years the location of these refuges has been forgotten by people living within the Great Barrier. That is why these places are now known as the Lost Caverns." "And the invaders still rule?" "Their descendants are still Lords of Gral-Thala. Cruel and ruthless they always were, decadent and dissolute they have now become as well, but they still rule the sunny valley that was the pride of the ancient Lunarians. They hold the power, and they are aided by a few groups among the people of Gral-Thala who have sacrificed their honor to fawn upon their masters. Our spies, who penetrate beyond the barrier, tell us that before long there will come a day when the people are ready for revolt--but the time is not yet." "But surely!" said Pyatt of Kagan, his deep voice breaking in on the low monotone in which Staunton had spoken, "surely our visitors will return to Earth, now that interplanetary travel has become possible, and bring us the warriors and equipment to storm the high palaces of the tyrants of Gral-Thala!"