See how evenly this road has been worn and it is leading toward
the centre of the island."

In a few 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.