Stanton looked at me oddly.

"'So you felt it too,' he said. 'I was wondering whether I was
developing hallucinations like Thora. Notice, by the way, that the
blocks beside it are quite warm beneath the sun.'

"We examined the slab eagerly. Its edges were cut as though by an
engraver of jewels. They fitted against the neighbouring blocks in
almost a hair-line. Its base was slightly curved, and fitted as
closely as top and sides upon the huge stones on which it rested. And
then we noted that these stones had been hollowed to follow the line
of the grey stone's foot. There was a semicircular depression running
from one side of the slab to the other. It was as though the grey rock
stood in the centre of a shallow cup--revealing half, covering half.
Something about this hollow attracted me. I reached down and felt it.
Goodwin, although the balance of the stones that formed it, like all
the stones of the courtyard, were rough and age-worn--this was as
smooth, as even surfaced as though it had just left the hands of the
polisher.

"'It's a door!' exclaimed Stanton. 'It swings around in that little
cup. That's what makes the hollow so smooth.'

"'Maybe you're right,' I replied. 'But how the devil can we open it?'

"We went over the slab again--pressing upon its edges, thrusting
against its sides. During one of those efforts I happened to look
up--and cried out. A foot above and on each side of the corner of the
grey rock's lintel was a slight convexity, visible only from the angle
at which my gaze struck it.