Journeying thus at top speed toward that hill of promise before them, the travelers came at length to a wide space of absolutely level ground which presented a most curious appearance. It was as level as a windless lake, and almost without vegetation. The naked surface was of a sort of indeterminate dust-color, but dotted here and there with tiny patches of vegetation so stunted that it was little more than moss. Grom, with his inquiring mind, would have liked to stop to investigate this curious surface, unlike anything he had ever seen before. But the hordes of the sambur were behind, pressing the tribe onwards, and straight ahead was the wooded hill, dense with foliage, luring with its promise of safe and convenient shelter. He led the way, therefore, without hesitation, out across the baked and barren waste, sniffing curiously, as he went, at a strange smell, pungent but not unpleasant, which steamed up from the dry, hot surface all about him. The first peculiarity that he noticed was a remarkable springiness in the surface upon which he trod. Then he was struck by the fact that the dust-brown surface was seamed and criss-crossed in many places by small cracks--like those in sun-scorched mud, except that the cracks were almost black in color. These things caused him no misgivings. But presently, to his consternation, he detected a slight but amazing undulation, an immensely long, immensely slow wave rolling across the dry surface before him. He could hardly believe his eyes--for assuredly nothing could look more like good solid land than that stretch of barren plain. He stopped short, rubbing his eyes in wonder. A-ya grabbed him by the arm. "What is it?" she whispered, staring at the unstable surface in a kind of horror. Before he could reply, cries and shouts arose among the tribe behind him, and they all rushed forward, almost sweeping Grom and A-ya from their feet. The surface of the barren, all along the edge of the grass land, had given way beneath the weight of the sambur herds, and the front ranks were being engulfed with frantic snortings and awful groans, in what looked like a dense, blackish, glistening ooze. The ranks behind were being forced forward to this awful doom, in spite of their panic-stricken struggles to hold back; and it was the pressure of this battling mass that was creating the horrible, bulging undulation on the plain. Grom's quick intelligence took in the situation on the instant. The naked brown surface beneath the feet of the tribe was nothing more than a thin crust overlying a lake of some dense, dark, strange-smelling liquid. His first impulse, naturally, was to turn back--and A-ya, with wide eyes of terror, was already dragging fiercely at his elbow. But to turn back was utterly impossible. That way lay the long strip of engulfing pitch, swallowing up insatiably the ranks of the groaning and kicking sambur. There was but one possible way of escape left open, and that was straight ahead.