"Well, yeah." It's a ßehemoth filter, obviously. And it must be blocking a whole range of other particles too, to generate this kind of density imbalance. "What I mean is, we can just punch right on through."

"I don't think so," Lubin says.

He brings the periscope down from the surface and sends it sniffing towards the barrier; on the panel, the cowering cityscape disappears in a swirl of bubbles and darkness. Clarke glimpses the 'scope's tether through the viewport, a pale thread of fiberop unwinding overhead. The periscope itself is effectively invisible, a small miracle of dynamic countershading.

Clarke watches it on tactical instead. Lubin brings the drone to within half a meter of the membrane: a faint yellow haze resolves on the right-hand feed, where naked eyes see only darkness. "What's that?" Clarke wonders.

"Bioelectric field," Lubin tells her.

"You mean it's alive?"

"Probably not the membrane itself. I'd guess it's run through with some kind of engineered neurons."

"Really? You sure?"

Lubin shakes his head. "I'm not even sure it's biological—the field strength fits, but it doesn't prove anything." He gives her a look. "Did you think we had a sensor to pick up brain cells at fifty paces?"

No witty rejoinder springs to mind. Clarke turns back to the viewport, and the dim blue aurora flickering beyond. "Like an anorexic smart gel," she murmurs.

"Probably a lot dumber. And a lot more radical—they'd have to tweak the neurons to work at low temperatures, high salinity—the membrane itself could handle osmoregulation, I suppose."

"I don't see any blood vessels. I wonder how they get nutrients."

"Maybe the membrane handles that too. Absorbs them right from seawater."

"What's it for?"

"Other than a filter?" Lubin shrugs. "An alarm, I should think."

"So what do we do?"

Lubin considers a moment. "Poke it," he says.

The periscope lunges forward. On the wide-spectrum display the membrane flares on impact, bright threads radiating from the strike like a fine-veined tracery of yellow lightning. In visible light it just floats there, inert.

"Mmm." Lubin pulls the periscope back. The membrane reverts to lowglow.

"So if it is an alarm," Clarke says, "I'm guessing you've just set it off."

"Not unless Halifax goes to red alert every time a piece of driftwood bumps their perimeter." Lubin runs his finger along a control bar: on tactical, the periscope heads back to the surface. "But I am willing to bet this thing'll scream a lot louder if we actually tear through it. We don't need that kind of attention."

"So what now? Head down the coast a bit, try a land approach?"

Lubin shakes his head. "Underwater was our best shot. A landside approach will be a lot tougher." He grabs a headset off the bulkhead and slips it over his skull. "If we can't get to a hard line, we'll try the local wireless nets. Better than nothing."