ret! A colossal secret!" he muttered, when I had ended.
"We cannot leave it hidden."

"The first thing to do is to try the door," said Larry, matter of
fact.

"There is no use, my young friend," assured Marakinoff mildly.

"Nevertheless we'll try," said Larry. We retraced our way through the
winding tunnel to the end, but soon even O'Keefe saw that any idea of
moving the slab from within was hopeless. We returned to the Chamber
of the Pool. The pillars of light were fainter, and we knew that the
moon was sinking. On the world outside before long dawn would be
breaking. I began to feel thirst--and the blue semblance of water
within the silvery rim seemed to glint mockingly as my eyes rested on
it.

"Da!" it was Marakinoff, reading my thoughts uncannily. "Da! We will
be thirsty. And it will be very bad for him of us who loses control
and drinks of that, my friend. Da!"

Larry threw back his shoulders as though shaking a burden from them.

"This place would give an angel of joy the willies," he said. "I
suggest that we look around and find something that will take us
somewhere. You can bet the people that built it had more ways of
getting in than that once-a-month family entrance. Doc, you and Olaf
take the left wall; the professor and I will take the right."

He loosened one of his automatics with a suggestive movement.

"After you, Professor," he bowed, politely, to the Russian. We parted
and set forth.

The chamber widened out from the portal in what seemed to be the arc
of an immense circle. The shining walls held a perceptible curve, and
from this curvature I estimated that the roof was fully three hundred
feet above us.

The floor was of smooth, mosaic-fitted blocks of a faintly yellow
tinge. They were not light-emitting like the blocks that formed the
walls. The radiance from these latter, I noted, had the peculiar
quality of _thickening_ a few yards from its source, and it was this
that produced the effect of misty, veiled distances. As we walked, the
seven columns of rays streaming down from the crystalline globes high
above us waned steadily; the glow within the chamber lost its
prismatic shimmer and became an even grey tone somewhat like moonlight
in a thin cloud.

Now before us, out from the wall, jutted a low terrace. It was all of
a pearly rose-coloured stone, slender, graceful pillars of the same
hue. The face of the terrace was about ten feet high, and all over it
ran a bas-relief of wha