The Hollow Hills are land that stopped completing itself, and it is not the only thing that never got finished. Here and there the world holds a place that obviously should have something in it and does not — and anybody can stand on the gap and look at it.
Closing one takes a memory. Not money, not materials alone: the knowing of how it went. Find out how, bring what it takes, and it is built — and it stays built, for everybody, permanently. Only one person ever has to know.
The first is the ford on the Keeper's road. Two dressed abutments face each other across the water on the way to the Ward Beacon, with nothing between them. Wade it and you cross with your shield down, which is a poor way to arrive at the Gloomline. Somebody once counted that bridge on a stick and dropped the stick.
The second is the cut that stops short, past the seam in the flooded gallery at Greyridge. The pump empties that gallery for about a minute and the hills fill it straight back — because the cut that should carry the water away runs twenty feet and stops, tool marks still on the face. Open it and the gallery is dry for good. Nothing appears; a place simply stops filling up.
The third is the Pale Furrow, and it is the one that changes the map. The Sainted did not wall the Gloom — they ploughed against it, a shallow trench along the line of it sown with a pale seed, and where the furrow runs the murk stands off. Three lengths of it survive, every one stopping mid-stride. Plough all three and the Gloomline comes back a hundred and twenty metres: ground that was murk this morning is the Hallow'd tonight. It can never take the whole thing — the Hills stay.
And once the line is three furrows back, a person can stand in the middle of the Hollow Hills and think, which is where the Right Name is remembered. Saying it costs nothing and moves nothing: the murk stays, the things in it stay, and the grass underneath stops looking like something that gave up. A name is not an eviction.
Two more change how far you can get. The Names of the Roads is what the Sainted did instead of marking them: they named their roads and taught the names to children, and the milestones are worn blank while the rhyme is still being skipped to in the lane. Know them and the roads appear on your map — and the going is easier where they run, because they are still there under the grass. The Deepcut is why every shaft in every mine stops at the same depth: there is a seam down there that does not break, it answers. Cut it on the slant and keep it wet, and what is under it is thick with green.
Not every memory finishes a place. Two of them are rules rather than blueprints. The Second Fire is how a flame is fed rather than struck, and anyone who knows it can keep a torch alight in a village the dark has got into — though not in the Gloom itself, which was never ours to light. The Quiet Hour is the hour before dawn, when the Unfinish'd stop noticing: it happens whether or not anybody knows it, and what the knowing buys is being told the moment it starts.