"Whatever is going to happen?" cried Billy in dismay, at the animals' evident terror of some mysterious event that was about to transpire, and the now marked disturbance of the earth.
As he spoke, the earth shook violently once more and a rumbling sound like subterranean thunder filled the air.
"It's the mountain!" shouted the professor, who had been gazing about, "it's going to erupt."
From the crater they had explored there were now rolling up great masses of bright, yellow smoke in sharp contrast to the dark vapors that had hitherto poured from it. A mighty rumbling and roaring proceeded from its throat as the smoke poured out, and vivid, blue flames shot through the sulphurous smother from time to time.
"We've no time to lose," cried Frank, "come on, we must get to the aeroplane in a hurry."
They all took to their heels over the trembling ground, not stopping to gaze behind them. The monsters had all disappeared, and as they had not been seen to re-enter their holes they were assumed to be hiding at the bottom of the lake.
As the boys gained the aeroplane and clambered in, Frank uttered an exclamation:
"Where's the professor?"
In a few seconds they espied him carefully bending over the dead body of one of the slain monsters several yards away.
"Come on, professor," they shouted, "there's no time to lose."