"Light both your lamps," exclaimed the professor, "and pick your footsteps. Remember, you are about to tread on strange ground." The professor led the way, his armour-clad figure looming up black and gigantic against the two overlapping discs of illuminated water before him, and the other three followed closely in his footsteps. On emerging from the trap-door they turned sharp to the left, and made their way toward the bow along the tunnel-like passage between the ship's bottom and the starboard bilge keel. This was soon traversed, and they then found themselves on a tolerably firm, level, gravelly bottom. Emerging from underneath the ship's bottom, they now extinguished their lamps for a moment by way of experiment, and found that so clear was the water that even at the great depth of ninety fathoms it was not absolutely dark, a sombre greenish blue twilight prevailing in which the hull of the ship towered above them vast and shadowy, yet with tolerable distinctness. This twilight, however, was strongly illuminated at both ends of the ship by the powerful electric lamps at the bow and stern, all of which the professor had taken the precaution to light before descending to the diving chamber. "Those are our beacons," said the professor, pointing to these lamps, "and we must be exceedingly careful not to stray beyond the reach of their rays, otherwise we might experience great difficulty in finding our way back to the ship. Are you all pretty comfortable in this great depth of water? We are now five hundred and forty feet beneath the surface of the sea, or three hundred and thirty-six feet deeper than man has ever reached before. Why, if we were to accomplish nothing more than this, we have already achieved a great triumph! Now, let us make our way toward the deepest spot in this submarine valley; I have an idea that we shall see something curious when we reach it. This way, gentlemen; our course is about due west, and we cannot well lose our way if we descend the slope which seems to commence yonder."