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 beyond it?
What did that smooth stone, whose wan deadness whispered of ages-old corridors of time opening out into alien, unimaginable vistas, hide?
It had cost the world of science Throckmartin's great brain--as it had cost Throckmartin those he loved.