But wait! From the southwest he hears a new echo that did not occur in his earlier experiment. It returns after two seconds and thus apparently comes from an obstruction 1000 feet away. Puzzled, the man decides to walk toward the object and check his observation. After he has covered half the distance he stops, sends out a call, and listens for the echo. The indicated distance to the echo-producing object is now 500 feet, just as he calculated. And so he goes on, checking at intervals. When he has covered 990 feet he knows that he should reach the obstruction at any moment and to avoid colliding with it in the darkness he proceeds with extreme caution--995, 996, 997, 998, 999 feet. He puts out his hand, expecting to touch a building or a stone wall, and warily takes the last step. But he finds no structure of any kind, merely level ground. And at the same moment he finds to his astonishment that he can no longer detect the echoes he had been following. What has happened? Has his equipment been malfunctioning? Or was the unknown structure perhaps a vehicle from outer space that waited until he was practically touching it and then rose silently in an enormous burst of speed and vanished? The man checks and finds that his equipment is functioning perfectly, since he can still pick up echoes from the terrain he had mapped earlier. He then walks back ten feet and listens once more for an echo from the phantom structure. Again he gets a signal, apparently from an obstruction just ten feet ahead. Has the mysterious object suddenly returned? But how could it have done so without disturbing the atmosphere or making a noise? By this time our man is frightened as well as puzzled, but he boldly decides to make one more experiment. He walks again to the point where the obstruction should be. Signaling again to the southwest, he now gets a faint echo apparently from a distance of 10,000 feet. Tired as he is, he starts walking toward this new obstruction and eventually reaches his goal. He now finds the true source of the returns--a high hill that rises abruptly from the plain. The hill is 10,000 feet away from the position indicated by the original series of echoes, and 11,000 feet away from the place he stood when he first sent out the signals.