They had. And they strangely resembled our own rock drills: revolving metal shafts, driven by gas turbines, tipped with fragments of the same crystal that glittered so profusely in the palace walls. Another proof that practically every basic, badly needed tool had been invented again and again, in all lands and times, as the necessity for it arose. With hundreds of the powerful men of Zyobor working as closely together as they could without cramping each others movements, and with the whole city resounding to the roar of the machinery, we labored at the defence that might possibly check the advance of the hideous Quabos. And with every breath we drew, waking or sleeping, we realized that the cold blooded, inhuman invaders had crept a fraction of an inch closer in their tunneling. The Quabos against the Zyobites! Fish against man! Two diametrically opposed species of life in a struggle to the death! Which of us would survive? * * * * * The hour of the struggle approached. Every soul in Zyobor moved in a daze, with strained face and fear haunted eyes. Their proficiency in mental telepathy was a curse to them now: every one carried constantly, transmitted from the brains of the servant-fish outposts, a thought picture of that outer cavern in the murky depths of which writhed the thousands of crowding Quabos. Each mind in Zyobor was in continual torment. Spared that trouble, at least, Stanley and the Professor and I walked down to the fortification we had so hastily contrived. It was finished. And none too soon: the vibration indicator in the palace vault told us that only two feet of rock separated us from the burrowing monsters! The Professor's scheme had been to cut a long slot down through the rock floor of the city to the roof of the vast, mysterious body of water below. This slot was placed directly in front of the spot in the city wall where the Quabos were about to emerge. As they forced through the last shell of rock, the deluge of water, instead of drowning the city, was supposed to drain down the oblong vent. Any Quabos that were too near the tunnel entrance would be swept down too. * * * * * In silence we approached the edge of the great trough and stared down. There was a stratum of black granite, fortunately only about thirty feet thick at this point, and then--the depths! A low roar reached our ears from far, far beneath us. A steady blast of ice cold air fanned up against us. The Professor threw down a large fragment of rock. Seconds elapsed and we heard no splash. The unseen surface was too far below for the noise of the rock's fall to carry on up to us.