‘A shaft,’ Gibson said. ‘Cut in the ground. Walls smooth to within the tolerance we can measure, and coated with some kind of – material – that we can’t make any sense of. And the depth, well, we can’t measure that either. Far beyond any depth where such a shaft ought to collapse in on itself.’ ‘And how wide? From space, the scale fooled me.’ ‘A hundred kilometres on a side, Michael. Near enough. And it’s regular – a perfect square. When you look at it from space it doesn’t look so odd, just a big hole in the ground. But down here – Michael, Mercury’s horizon is only three kilometres away. The far side of this shaft is so far away that you’re looking down to see it, by a degree or two, below the local horizontal. This is a well so big it cuts through the planet’s curvature.’
-Xeelee: Vengeance, Ch. 14