However, just when the delightful days were beginning to pall upon us, a real adventure befell us, which, had we been attending strictly to business, we should not have encountered.  For a week previous we had been cruising constantly without ever seeing a spout, except those belonging to whales out at sea, whither we knew it was folly to follow them.  We tried all sorts of games to while away the time, which certainly did hang heavy, the most popular of which was for the whole crew of the boat to strip, and, getting overboard, be towed along at the ends of short warps, while I sailed her.  It was quite mythological—­a sort of rude reproduction of Neptune and his attendant Tritons.  At last, one afternoon as we were listlessly lolling (half asleep, except the look-out man) across the thwarts, we suddenly came upon a gorge between two cliffs that we must have passed before several times unnoticed.  At a certain angle it opened, disclosing a wide sheet of water, extending a long distance ahead.  I put the helm up, and we ran through the passage, finding it about a boat’s length in width and several fathoms deep, though overhead the cliffs nearly came together in places.  Within, the scene was very beautiful, but not more so than many similar ones we had previously witnessed.  Still, as the place was new to us, our languor was temporarily dispelled, and we paddled along, taking in every feature of the shores with keen eyes that let nothing escape.  After we had gone on in this placid manner for maybe an hour, we suddenly came to a stupendous cliff—­that is, for those parts—­rising almost sheer from the water for about a thousand feet.  Of itself it would not have arrested our attention, but at its base was a semicircular opening, like the mouth of a small tunnel.  This looked alluring, so I headed the boat for it, passing through a deep channel between two reefs which led straight to the opening.  There was ample room for us to enter, as we had lowered the mast; but just as we were passing through, a heave of the unnoticed swell lifted us unpleasantly near the crown of this natural arch.  Beneath us, at a great depth, the bottom could be dimly discerned, the water being of the richest blue conceivable, which the sun, striking down through, resolved into some most marvellous colour-schemes in the path of its rays.  A delicious sense of coolness, after the fierce heat outside, saluted us as we entered a vast hall, whose roof rose to a minimum height of forty feet, but in places could not be seen at all.  A sort of diffused light, weak, but sufficient to reveal the general contour of the place, existed, let in, I supposed, through some unseen crevices in the roof or walls.  At first, of course, to our eyes fresh from the fierce glare outside, the place seemed wrapped in impenetrable gloom, and we dared not stir lest we should run into some hidden danger.  Before many minutes, however, the gloom lightened as our pupils enlarged, so that, although the light was faint, we could find our way about with ease.  We spoke in low tones, for the echoes were so numerous and resonant that even a whisper gave back from those massy walls in a series of recurring hisses, as if a colony of snakes had been disturbed.