That naked-eye visual occupies the upper-left quarter of the main panel. Beside it, a false-color interpretation of the same view shows a fuzzy, indistinct cloud enveloping the lit buildings; Clarke thinks of the mantle of a jellyfish, enclosing vital organs. The shroud is largely invisible to human eyes, even rifter ones; to Phocoena's spectrum-spanning senses, it looks like a blue haze of heat lightning. Static-field ionization, Lubin says. A dome of electricity to keep airborne particles at bay.

The seaward frontier is under guard. Not that Clarke ever expected to simply sneak into the harbor and pull up next to the local clam shack; she knew there'd be some kind of security in place. Lubin was expecting mines, so for the last fifty klicks Phocoena crawled towards the coast behind a couple of point drones zig-zagging ahead, luring any countermeasures out of concealment. Those flushed a single burrower lying in wait; awakened by the sound of approaching machinery, it shot from the mud and corkscrewed into the nearest drone with a harmless and anticlimactic clunk.

That lone dud was the only countermeasure they came across on the outer slope. Lubin figures that Halifax's subsurface defenses must have been used up fending off previous incursions. The fact that they haven't been replenished doesn't bode well for the mass-production of industrial goods in the vicinity.

At any rate, against all expectations they've cruised unchallenged all the way here, just outside Halifax Harbor. Only to nearly run into this. Whatever this is.

It's virtually invisible in the sub's lights. It's even less visible to sonar, which can barely pick it up even at point-blank range. A transparent, diaphanous membrane stretches from seabed to surface: the periscope shows a float line holding its upper edge several meters above the waves. It appears to stretch across the entire mouth of the harbor.

It billows inward, as if the Atlantic is leaning on it from the outside. Pinpoint flashes of cold blue light sparkle across its face, sparse ripples of stardust echoing the gentle subsurface surge. Clarke recognizes the effect. It's not the membrane that sparkles, but the tiny bioluminescent creatures colliding with it.

Plankton. It seems somehow encouraging that they still exist, so close to shore.

Lubin's less interested in the light show than its cause. "Must be semipermeable." That would explain the oceanographic impossibility that belied its presence, a sudden sharp halocline rising across their path like a wall. Discrete boundaries are common enough in the sea: brackish water lying atop heavier saline, warm water layered over cold. But the stratification is always horizontal, a parfait of light-over-heavy as inevitable as gravity. A vertical halocline seems to violate the very laws of physics; the membrane itself may have been undetectable to sonar, but the sheer knife-edged discontinuity it produces showed up like a brick wall from a thousand meters away.

"Looks pretty flimsy," Clarke remarks. "Not much to keep us out."

"It's not there for us," Lubin says.