After what felt like a long, long time, Eshu heard a creak of branches and a rustling of leaves. A few minutes later, Fern dropped to the ground a little way from him. “There’s a mirror, way over there,” she said, and she pointed in a direction that looked the same as any other. “It’s in the bottom of an arroyo, but you can see it from the top of the tree. A sort of oval of light.”

Eshu wasn’t sure what an arroyo was, but Fern seemed to know, and he believed her.

They set out together over the sands, Fern enthusiastically speculating on the minerals that composed the black ridges and the way the silver trees might have spread and sustained themselves. “It’s obvious that there’s a spring somewhere in the mountains, or possibly several springs,” she said. “My guess is that this black rock is just porous enough to let water drain through, and there’s a major aquifer somewhere below our feet—”

“It’s a dreamscape,” said Eshu. He wished he didn’t sound quite so desperate. “You can’t just apply science to it.”

“Then explain the arroyos. They’re all more or less perpendicular to the mountains. I wonder what the rain is like here.”

“It never rains here. The sun never rises. It’s a void beyond time and space.”

“It’s hydrologically interesting.”

Despite himself, Eshu took some relief in the bickering. The cold and darkness of the desert seemed much less terrible when Fern was darting ahead to collect rock and leaf samples or pausing to correct the scale on a hand-drawn map. She seemed to love the eccentricities of the Mirrorlands, and not to be unsettled by them. She made them seem like a place that could be known.

After what felt like an hour or two of walking, the faint gully they were following opened into a valley of black gravel and low stands of silver trees. There was water here, just a trickle almost too small to call a stream, and Fern stoppered a little in a glass vial. “I might find a microscope eventually,” she said, when Eshu gave her a look. “I want to see if there are tiny creatures in it.”

At the end of the valley, on a stone that jutted up from the gravel bed, there hung a mirror that shone with a cool, silvery light. Eshu’s heart leapt at the sight of it. “There’s no cognitive locus,” he said, more to himself than to Fern. “No way of knowing where this leads.”

“Well, then we’ll just have to walk through.”

“Is walking how geographers solve all of their problems?”

“Anything we can’t solve with trigonometry!” she said brightly. Then, visibly nerving herself up, she stepped through the oval of light and into the world beyond.

A hungry wind hissed through the boughs of the silver trees. Eshu cast an anxious look around at the valley—the arroyo—and then followed Fern through.

He emerged into a crystal-lined cavern, so cramped that he had to stoop to keep from hitting his head on the rocks hanging down from the ceiling. The mirrorlight shone on massive gemstones that erupted from every surface in cracked cubes and angles, all of them swirled with violet and blue and deep, translucent green. Amethyst? he thought. No—fluorite. He could dimly remember attending a guest lecture once on the magical properties of crystals, but in Usbaran, his cohort had regarded that sort of thing as superstitious quackery. He’d never bothered to learn whether fluorite had any uses.

“Eshu?” said Fern. She pointed to a square on one wall where the crystals glowed with a strange, bright light. “I think that used to be a window.”