ew minutes the two adventurers came to a point in the road where
tall cliffs on either side stood up perpendicularly. It was dark and
cold, and but a faint light from the moon shone down to them.

"I don't like this," said Johnston, who was behind the Englishman; "we
may be walking into the ambush of an enemy."

"Pshaw!" and Thorndyke plunged on into the gloomy passage. Presently the
walls began to widen like a letter "Y" and in a great open space they
saw a placid lake on the bosom of which the moon was shining. On all
sides the towering walls rose for hundreds of feet. Speechless with
wonder and with quickly-beating hearts they stumbled forward over the
uneven road till they reached the shore of the lake. The water was so
clear and still that the moon and stars were reflected in it as if in a
great mirror.

"Look at that!" exclaimed Thorndyke, pointing down into the depths,
"what can that be?"

Johnston followed Thorndyke's finger with his eyes. At first he thought
that it was a comet moving across the sky and reflected in the water;
but, on glancing above, he saw his mistake. It looked, at first, like a
great ball of fire rolling along the bottom of the lake with a stream of
flame in its wake.

Chapter II.

The two men watched it for several minutes; all the time it seemed to be
growing larger and brighter till, after a while, they saw that the light
came from something shaped like a ship, sharp at both ends, and covered
with oval glass. As it slowly rose to the surface they saw that it
contained five or six men, sitting in easy chairs and reclining on
luxurious divans. One of them sat at a sort of pilot-wheel and
was directing the course of the strange craft, which was moving as
gracefully as a great fish.

Then the young men saw the man at the pilot-wheel raise his hand,
and from the water came the musical notes of a great bell. The vessel
stopped, and one of the men sprang up and raised an instrument that
looked like a telescope to his eyes. With this he seemed to be closely
searching the lake shores, for he did not move for several minutes. Then
he lowered the instrument, and when the bell had rung again, the vessel
rose slowly and perpendicularly to the surface and glided to the shore
within twenty yards of where the adventurers stood.

"Could they have seen us?" whispered Thorndyke, drawing Johnston nearer
the side of the cliff.

"I think so; at all events, they are between us and the outlet; we