The air seemed to have grown more close and sultry while the children were within the cave, and, just as they reached the mouth, a low, grumbling sound, which they had heard for some time, but not taken much notice of, broke into a crash of thunder that seemed to make the ground shake beneath them, while at the same instant a brilliant streak of lightning flashed zigzag across the sky, lighting up the gloom behind them to its furthest recess.