“ ‘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.