tally and david left at sunset.

each of them rode two hoverboards. pressed togetherlike a sandwich, the paired boards could carry twice asmuch weight, most of it in saddlebags slung on the underside.

they packed everything useful they could find, alongwith the magazines the boss had saved. whatever happened,there would be no point in returning to the smoke.

tally took the river down the mountain carefully, theextra weight swaying below her like a ball and chain aroundboth ankles. at least she was wearing crash bracelets again.

their journey would follow a path very different fromthe one tally had taken there. that route had been designedto be easy to follow, and had included a helicopter ride withthe rangers. this one wouldn’t be as direct. overloaded asthey were, tally and david couldn’t manage even short distanceson foot. every inch of the journey had to be overhoverable land and water, no matter how far it took themout of their way. and after the invasion, they would be givingany cities a wide berth.

fortunately, david had made the journey to and fromtally’s city dozens of times, alone and with inexperienceduglies in tow. he knew the rivers and rails, the ruins andnatural veins of ore, and dozens of escape routes he’ddevised in case he was ever pursued by city authorities.

“ten days,” he announced when they started. “if weride all night and stay low during the day.”

“sounds good,” tally said, but she wondered if thatwould be soon enough to save anyone from the operation.

around midnight the first night of travel, they left thebrook that led down to the bald-headed hill, and followeda dry creek bed through the white flowers. it took them tothe edge of a vast desert.

“how do we get through that?”

david pointed at dark shapes rising up from the sand,a row of them receding into the distance. “those used to betowers, connected by steel cables.”

“what for?”

“they carried electricity from a wind farm to one of theold cities.”

tally frowned. “i didn’t know the rusties used windpower.”

“they weren’t all crazy. just most of them.” heshrugged. “you’ve got to remember, we’re mostlydescended from rusties, and we’re still using their basictechnology. some of them must have had the right idea.”

the cables still lay buried in the desert, protected byuglies 341the shifting sands and a near-total absence of rainfall. inspots, they had broken or rusted through, so tally anddavid had to ride carefully, eyes glued to the boards’ metaldetectors. when they reached a gap they couldn’t jump,they would unroll a long piece of cable david carried, thenwalk the boards along it, guiding them like reluctant donkeysacross some narrow footbridge before rolling it upagain.

tally had never seen a real desert before. she’d beentaught in school that they were full of life, but this one waslike the deserts she’d imagined as a littlie—featurelesshumps stretching into the distance, one after another.

nothing moved but slow snakes of sand borne by thewind.

she only knew the name of one big desert on the continent.

“is this the mojave?”

david shook his head. “this isn’t nearly that big, andit isn’t natural. we’re standing where the white weedstarted.”

tally whistled. the sand seemed to go forever. “what adisaster.”

“once the undergrowth was gone, replaced by theorchids, there was nothing to hold the good soil down. itblew away, and all that’s left is sand.”

“will it ever be anything but desert?”

“sure, in a thousand years or so. maybe by then someonewill have found a way to stop the weed from coming342 scott westerfeldback. if we haven’t, the process will just start all overagain.”

they reached a rusty city around daybreak, a cluster ofunremarkable buildings stranded on the sea of sand.

the desert had invaded over the centuries, dunes flowingthrough the streets like water, but the buildings were inbetter shape than other ruins tally had seen. sand woreaway the edges of things, but it didn’t tear them down ashungrily as rain and vegetation.

neither of them was tired yet, but they couldn’t travelduring the day; the desert offered no protection from thesun, nor any concealment from the air. they camped in thesecond floor of a low factory building that still had most ofits roof. ancient machines, each as big as a hovercar, stoodsilent around them.

“what was this place?” tally asked.

“i think they made newspapers here,” david said.

“like books, but you threw them away and got a new oneevery day.”

“you’re kidding.”

“not at all. and you thought we wasted trees in thesmoke!”

tally found a patch of sun shining through where theroof had collapsed, and unfolded the hoverboards torecharge. david pulled out two packets of eggsal.

“will we make it out of the desert tonight?” she asked,uglies 343watching david coax their last few drops of bottled waterinto the purifiers.

“no problem. we’ll hit the next river before midnight.”

she remembered something that shay had said a longtime ago, the first time she’d shown tally her survival gear.

“can you really pee in a purifier? and then drink it, imean?”

“yeah. i’ve done it.”

tally grimaced and looked out the window. “okay, ishouldn’t have asked.”

he came up behind her, laughing softly, placing hishands on her shoulders. “it’s amazing what people will doto survive,” he said.

she sighed. “i know.”

the window overlooked a side street, partly protectedfrom the encroaching desert. a few burned-out groundcarsstood half-buried, their blackened frames stark against thewhite sand.

she rubbed the handcuff bracelets still encircling herwrists. “the rusties sure wanted to survive. every ruin i’veseen, those cars are always all over, trying to get out. butthey never seem to make it.”

“a few of them did. but not in cars.”

tally leaned back into his reassuring warmth. themorning sun was hours away from burning off the chill ofthe desert.

“it’s funny. at school, they never talk much about how344 scott westerfeldit happened—the last panic, when the rusty world fellapart. they shrug and say that all their mistakes just keptadding up, until it all collapsed like a house of cards.”

“that’s only partly true. the boss had some old booksabout it.”

“what did they say?”

“well, the rusties did live in a house of cards, butsomeone gave it a pretty big shove. no one ever found outwho. maybe it was a rusty weapon that got out of control.

maybe it was people in some poor country who didn’t likethe way the rusties ran things. maybe it was just an accident,like the flowers, or some lone scientist who wanted tomess things up.”

“but what happened?”

“a bug got loose, but it didn’t infect people. it infectedpetroleum.”

“oil got infected?”

he nodded. “oil is organic, made from old plants anddinosaurs and stuff. somebody made a bacterium that ateit. the spores spread through the air, and when they landedin petroleum, processed or crude, they sprouted. like amold or something. it changed the chemical composition ofthe oil. have you ever seen phosphorus?”

“it’s an element, right?”

“yeah. and it catches fire on contact with air.”

tally nodded. she remembered playing with the stuff inchem class, wearing goggles and talking about all the tricksuglies 345you could do with it. but no one ever thought of a trick thatwouldn’t kill someone.

“oil infected by this bacterium was just as unstable asphosphorus. it exploded on contact with oxygen. and as itburned, the spores were released in the smoke, and spreadon the wind. until the spores got to the next car, or airplane,or oil well, and started growing again.”

“wow. and they used oil for everything, right?”

david nodded. “like those cars down there. they musthave been infected as they tried to get out of town.”

“why didn’t they just walk?”

“stupid, i guess.”

tally shivered again, but not from the cold. it was hardto think of the rusties as actual people, rather than as justan idiotic, dangerous, and sometimes comic force of history.

but there were human beings down there, whatever was leftof them after a couple of hundred years, still sitting in theirblackened cars, as if still trying to escape their fate.

“i wonder why they don’t tell us that in history class.

they usually love any story that makes the rusties soundpathetic.”

david lowered his voice. “maybe they didn’t want youto realize that every civilization has its weakness. there’salways one thing we depend on. and if someone takes itaway, all that’s left is some story in a history class.”

“not us,” she said. “renewable energy, sustainableresources, a fixed population.”

346 scott westerfeldthe two purifiers pinged, and david left her to get them.

“it doesn’t have to be about economics,” he said, bringingthe food over. “the weakness could be an idea.”

she turned to take her eggsal, cupping its warmth inher hands, and saw how serious he looked. “so, david, isthat one of the things you thought about all those years,when you imagined the smoke being invaded? did youever wonder what would turn the cities into history?”

he smiled and took a big bite.

“it gets clearer every day.”