"The most practical means of transportation that exists today for carrying out my plan would be a specially constructed rigid airship, built along the lines of the modern Zeppelin. Such a ship, using helium gas, would show a higher factor of safety than any other means of transportation at our disposal. I have given the matter considerable thought and I feel sure that if there is such a polar opening, the obstacles that would confront us in an attempt to enter the inner world would be far less than those encountered by the Norge in its famous trip across the pole to Alaska, for there is no question in my mind but that it made a wide detour in following the rim of the polar orifice and covered a far greater distance than we shall have to cover to reach a reasonably safe anchorage below the cold, polar sea that David Innes discovered north of the land of the Korsars before he was finally taken prisoner by them. "The greatest risk that we would have to face would be a possible inability to return to the outer crust, owing to the depletion of our helium gas that might be made necessary by the maneuvering of the ship. But that is only the same chance of life or death that every explorer and scientific investigator must be willing to assume in the prosecution of his labors. If it were but possible to build a hull sufficiently light, and at the same time sufficiently strong, to withstand atmospheric pressure, we could dispense with both the dangerous hydrogen gas and the rare and expensive helium gas and have the assurance of the utmost safety and maximum of buoyancy in a ship supported entirely by vacuum tanks." "Perhaps even that is possible," said Tarzan, who was now evincing increasing interest in Gridley's proposition. The American shook his head. "It may be possible some day," he said, "but not at present with any known material. Any receptacle having sufficient strength to withstand the atmospheric pressure upon a vacuum would have a weight far too great for the vacuum to lift." "Perhaps," said Tarzan, "and, again, perhaps not." "What do you mean?" inquired Gridley. "What you have just said," replied Tarzan, "reminds me of something that a young friend of mine recently told me. Erich von Harben is something of a scientist and explorer himself, and the last time that I saw him he had just returned from a second expedition into the Wiramwazi Mountains, where he told me that he had discovered a lake-dwelling tribe using canoes made of a metal that was apparently as light as cork and stronger than steel. He brought some samples of the metal back with him, and at the time I last saw him he was conducting some experiments in a little laboratory he has rigged up at his father's mission." "Where is this man?" demanded Gridley. "Dr. von Harben's mission is in the Urambi country," replied the ape-man, "about four marches west of where we now are."