blocks in the midst
of the sapphire sea, lay dozens of islets, none of them covering more
than two square miles of surface; each of them a perfect square or
oblong within its protecting walls.

On none was there sign of life, save for a few great birds that
hovered here and there, and gulls dipping in the blue waves beyond.

We turned our gaze down upon the island on which we stood. It was, I
estimated, about three-quarters of a mile square. The sea wall
enclosed it. It was really an enormous basalt-sided open cube, and
within it two other open cubes. The enclosure between the first and
second wall was stone paved, with here and there a broken pillar and
long stone benches. The hibiscus, the aloe tree, and a number of small
shrubs had found place, but seemed only to intensify its stark
loneliness.

"Wonder where the Russian can be?" asked Larry.

I shook my head. There was no sign of life here. Had Marakinoff
gone--or had the Dweller taken him, too? Whatever had happened, there
was no trace of him below us or on any of the islets within our range
of vision. We scrambled down the side of the gateway. Olaf looked at
me wistfully.

"We start the search now, Olaf," I said. "And first, O'Keefe, let us
see whether the grey stone is really here. After that we will set up
camp, and while I unpack, you and Olaf search the island. It won't
take long."

Larry gave a look at his service automatic and grinned. "Lead on,
Macduff," he said. We made our way up the steps, through the outer
enclosures and into the central square, I confess to a fire of
scientific curiosity and eagerness tinged with a dread that O'Keefe's
analysis might be true. Would we find the moving slab and, if so,
would it be as Throckmartin had described? If so, then even Larry
would have to admit that here was something that theories of gases and
luminous emanations would not explain; and the first test of the whole
amazing story would be passed. But if not--And there before us, the
faintest tinge of grey setting it apart from its neighbouring blocks
of basalt, was the moon door!

There was no mistaking it. This was, in very deed, the portal through
which Throckmartin had seen pass that gloriously dreadful apparition
he called the Dweller. At its base was the curious, seemingly polished
cup-like depression within which, my lost friend had told me, the
opening door swung.

What was that portal--more enigmatic than was ever sphinx? And what
lay be