by Joe Buff
"Uh-huh," Sessions said. "Several groups of gradiometers and accelerometers throughout the boat. They measure changes in mass concentrations from different bearings."
"And this is all continuous motion," Ilse said, "in real time?"
"Yeah, the same thing COB and Meltzer have right now. It's refreshed every ten seconds."
"It must use lots of processing power."
"It's worth it," Sessions said. "We've got a hundred times the original Seawolf-class computer capabilities. The basic gravimeter math's nonclassified, but ours has special stuff civilian geologists don't know about."
"How new is this?"
"They were testing one on USS Memphis back in the late nineties at DEVRON TWELVE, our squadron. That's what we do. We're operational SSNs and we also test technology and tactics, working with the Naval Underwater Systems Center in Rhode Island, and the various contractors."
Ilse stared at the terrain estimation display. "You can see right through things!"
"Sure," Sessions said. "Matter's transparent to gravity, right? One seamount shows up past another. Good thing too, else we couldn't go this fast so close to the bottom."
"You must use some kind of stored data as backup, don't you?"
"We do," Sessions said, "and to speed these calculations also. We have decent bottom charts, for gross verification and course plotting."
"You don't just feel your way?"
"Not usually. Hitting a basalt cliff head-on would be embarrassing."
"Yeah," Ilse said.
"The helm guys need to stay real sharp," Sessions said. "At twenty-six knots we move one boat length every eight seconds. Watch this." On-screen Ilse could see that the canyon they'd been following took a sharp turn to the left. As if on cue to her thoughts the boat banked to port. She watched as Challenger followed the canyon leftward, still hugging the deep ravine's right wall, several hundred feet up. The boat leveled off as it came out of the turn.
"You can see why we don't stream a towed array," Sessions said.
"Neat," Ilse said. "But how come we don't stay right on the bottom?"
"Stealth. It's too obvious. If we follow one wall partway up, we still get all the benefits of terrain masking."
"And I guess that makes it tougher for the enemy to lie in ambush or plant a mine."
"You got it, Ilse. It also gives us more lateral clearance, room for turning sideways just in case. Right down on the canyon floor we'd be boxed in."
"Can you use this under ice?" Ilse said. "For ice avoidance?"
"Maybe someday," Sessions said. "It's great to fix your posit under the ice cap, update your inertial nav, since you can't just pop up for a GPS fix then, even in peacetime. Instead we orienteer from the gravimeter, based on distinctive bottom features and our charts. For ice avoidance we have to use our sail-mounted high-frequency sonar, which radiates and has short range and can't see past a bummock or berg. The problem with the gravimeter is the density gradient's not strong enough — rock or sediment versus water's one thing, but ice versus water's something else."
"Then what about detecting other subs? Or surface ships?"
"Smart question," Sessions said. "No good, unfortunately, is the answer. Floating surface units and submarines displace a mass of water equal to their weight, right? So unless you're really close, there's not enough change in the gravity field."
"What if you are close?" Ilse said.
"There's another problem if they're moving," Sessions said. "It adds a centripetal gradient that's unknown to the algorithms, not like own-ship velocity. And if you do know target range and course and speed, who needs the gravimeter?"
"What about a nuclear sub, though, one that's motionless or on the bottom? The reactor compartment must be extra heavy. Wouldn't that show up, compared to spaces full of air, as a mass field discontinuity?"
"Now you're getting into classified stuff," Sessions said. "But it's no secret gravimetry can help you avoid something man-made that's real heavy, like an oil drilling platform, since it doesn't move."
Ilse went back to her screens. Watching Challenger's swift progress through the benthic topography was fascinating. The boat followed an S-curve between underwater peaks, hard right and then hard left again. Ilse could see it coming, on the bird's-eye view, and she watched the other picture as they took the turns, like looking out the windshield of a car.
"I have another question," Ilse said, "if this isn't secret."
"Try me," Sessions said.
"I need to understand this to help you navigate. How come Jeffrey isn't giving orders?"
"You mean no helm commands?"
"Yeah. When we fought those diesel boats, Captain Wilson kept saying make your course this, make your depth that, hard right rudder … "
"Nap-of-seafloor's different. That's one reason we have senior people at the helm. Commander Fuller can overrule them, even take control himself if need be from his console, but this is different from maneuvers near the surface. Here we follow the terrain."
"So our detailed course is a given," Ilse said. "Mother Nature calls the shots."
"Pretty much."
"Could you fight a battle this far down?"
"I suppose," Sessions said. "The enemy would have to find us first. Down here you get lost in the sonar grass, just like with radar, and even look-down acoustic Doppler, which tracks suspended particles, gets confused by multiple currents at different depths."
"So more depth means more protection?" Ilse said.
"Yeah," Sessions said. "It's not just that you get more thermal and salinity layers to hide behind—"
"Plus the deep scattering layer too," Ilse said, "as schools of biologics migrate up and down the water column every day."
"That's right," Sessions said. "Anyway, the point is, if the enemy is sort of overhead, the spherical attenuation model holds. The intensity of our self-noise as received by the other guy goes down with the square of range, so ten times deeper means just one percent the signal strength."
"I should bone up on this stuff," Ilse said.
"You need to," Jeffrey interrupted. "There's a series of tutorials you can run on the computer, with homework problems and everything."
Ilse realized he'd been listening in. There really is no privacy on a submarine, she told herself. "Yes, Commander," she responded, slightly irked. "I'll look at the tutorials once I'm done with the gravimeter." Jeffrey turned back to his console. Ilse watched her screens again. She saw large boulders show up now and then. Probably from underwater landslides, from seismic activity she knew never really ceased. Sometimes she saw talus slopes, rubble built up over eons at the base of undersea escarpments.
"You can fiddle with the picture if you like," Sessions said. "Use your trackmarble to look ahead, or rotate the presentation and see things from a different angle." He showed her how.
"This is fun," Ilse said. "But I have another question. With nap-of-seafloor, we're not too exposed, you know, to enemy sensors planted on the bottom?"
"Yes and no," Jeffrey butted in again. "That's why we use broken terrain, where it's hard to place a trip-wire grid and seafloor current eddies make for lots of false alarms. We'd avoid a smooth, flat open-ocean basin at all costs, for just that reason."
"Oh, okay," Ilse said.
"Another thing," Sessions said, "with the exotic nonacoustic ASW methods, like surface-wake anomalies, is they need minutes or hours of supercomputer time to make sense of the raw data, by which point you're tens of miles away. That's why we always zigzag these days, even out in blue water, deny the enemy our base course and speed."
"Right," Ilse said. Boy, do these guys love their shop talk.
"And now," Sessions said, "please excuse me." He got up and started walking the line of sonar consoles, talking to his people. Ilse went back to her screens. Challenger plunged onward.
Up ahead, on the display, she noticed an interesting formation of big rocks on the bottom, where the canyon opened out. The rocks lay in a perfect triangle.
"What's that, Helm?" she heard Jeffrey say.
"More boulders, sir," Meltzer said. "We'll pass well clear."
"Very well, Helm," Jeffrey said.
As they got closer to the boulders, Jeffrey watched the gravimeter's resolution sharpen. " That middle one. COB, if you will, gimme a close-up."
The image changed as Challenger seemed to leap ahead.
Jeffrey stared. "All stop!" he shouted. "Hover on manual!"
"All stop, aye," Meltzer said.
"Rig for ultraquiet," Jeffrey said.
"Aye, sir," COB responded instantly. The CACC lights blinked urgently and Sessions dashed back to his seat. Phone talkers hurried to their positions as Jeffrey reached for the handset to call the captain.
"Captain's in the CACC," the messenger announced. Jeffrey turned to see Wilson right behind him, in boxer shorts and slippers.
"What is it?" Wilson said.
Jeffrey pointed. "Those three objects on the bottom there. Around the next bend, range about one thousand yards. They look too much like subs."
"I see them," Wilson said.
"The closest one," Jeffrey said, "beam-on to us — that mass gradient can't be natural. I think we're seeing reactor shielding, Captain, and a core."
"Awfully big reactor for a vessel of that size," Wilson said. "Each of them's barely a hundred feet from stem to stern."
"Who'd be down here?" Jeffrey said. "Japanese?"
"In the war zone?" Wilson said. "They'd love to know what's going on, but they're not crazy."
"Something new the Axis has?" Jeffrey said.
"It's possible," Wilson said. "The other two could be ceramic diesel/AIPs."
"Or a clandestine seafloor habitat?" Jeffrey said. "For intel gathering maybe?"
"Maybe," Wilson said.
"The way they're all just sitting there, sir," Jeffrey said, "like they've circled as a laager Boer style, I don't like it."
"I don't either," Wilson said. "XO, I have the conn. Chief of the Watch, sound quiet general quarters. Man battle stations antisubmarine."
Jeffrey slid over, but Wilson stayed in the aisle, studying the helm screens. More crewmen hurried into the CACC and powered up their consoles.
"Captain," Jeffrey said, "recommend we get in closer, get some visuals, and do a full sound profile on these contacts. We can't just sneak around them and leave unknowns in our rear."
"I concur," Wilson said. "Fire Control, prepare to launch a long-range mine reconnaissance system vehicle."
"Prepare to launch an LMRS, aye," Jeffrey said. "Recommend we skip autonomous mode to stay covert."
"Concur," Wilson said. "Belay acoustic uplink, use the fiber-optic tether."
"Recommend we warm up the Mark 88 in tube three," Jeffrey said.
"Warm up the nuclear torpedo in tube three," Wilson said.
"Messenger of the Watch," Jeffrey said, "bring the captain some black coffee and his bathrobe, please."
Jeffrey concentrated on the screen, his right hand glued to the trackmarble. "Proton magnetometer's getting something, Captain. Field lines are bunching up."
"What's the range?" Wilson said.
"From the nearest mass concentration to our probe, four hundred yards."
"Get closer. Make it three."
Jeffrey tapped some keys, increasing the image intensification gain to a factor of 10, 000. Diffuse and point-source bioluminescence, blues and greens stirred up by the torpedo-shaped LMRS, gave him a fuzzy view for a few yards ahead of its nose-mounted charge-coupled eyes. Jeffrey gingerly piloted the probe up off the bottom, inching it closer to the enemy subs, using a dip in the ground for cover.
"Definitely ferrous hulls," he said a minute later, eyeing the magnetometer again.
"Take it to two hundred yards," Wilson said.
"Aye aye, sir," Jeffrey said. "I'll hang a left, see where that fissure goes."
"Sonar," Wilson almost whispered, "anything?"
"Slight flow noises from bottom currents, sir," Sessions said. "Nothing artificial."
"Okay," Wilson said, "they're meeting their hotel load off batteries or fuel cells, and cooling that reactor convectively or just letting it run hot." Jeffrey listened to the conversation as he gently moved the probe. "Ready now," he said.
"Pop up and take a look," Wilson said.
Jeffrey increased the CCD image gain once more, to 50,000 times. As the probe rose slowly, he saw the lip of the fissure moving down the screen.
"Whoa!!" Jeffrey ducked the probe back down, his heart pounding in his throat.
"What was that?" Wilson said.
"A bow dome, sir. Big." Jeffrey ran the replay. "Should we flood tube three?"
"Sonar," Wilson said, "any reaction?"
"Negative, sir," Sessions said. "No sign they know we're here."
"Should we flood tube three?"
"Negative," Wilson snapped. "They'll hear it. We have to back off first to fire anyway."
"Captain," COB interrupted, "that sub's not showing on the gravimeter at all."
"Or the probe's not where it should be," Jeffrey said. He checked his screens and enunciators. "Negative on a guidance flaw. Position overlay matches with dead reckoning." Again he brought the probe up. "Captain, look at this … a gigantic crack in the fiberglass."
The probe rose over the top of the enemy sonar dome. "It's just the cap," Jeffrey said. "There's nothing behind it." Wilson sputtered. "We found a goddamn wreck."
"A fresh one, sir," Jeffrey said. "No growths yet, and hardly any sea snow."
"All right," Wilson said. "Check out that middle contact now."
"Aye aye, sir," Jeffrey said. "I'll be careful — a wreck's the perfect spot to hide an ambush."
"Sonar," Wilson said, "what's happening?"
"No change, Captain."
Jeffrey moved the probe. "Something here." The murky picture showed a twisted, fractured, splintered mass.
"That used to be the sail," Wilson said. "See? There's the mounting for the port-side fairwater plane … What's left of it."
"I'll go around," Jeffrey said. Suddenly ahead of the probe there loomed another mass, a huge one. "The reactor compartment," he said.
"Get closer," Wilson said. "Switch to active line scan. Lowest emitter power." The picture changed, no longer an eerie natural glow. Now it was much sharper, more detailed. Jeffrey could see tattered steel, wires, and cables waving in the current. He saw a small dogged hatch in the middle of a bulkhead, next to broken pipes.
"The forward accessway," he said, "into the reactor tunnel." He brought the probe in closer, already suspecting. He could see the writing now, fragments of the safety warnings posted near the access door. "It's one of ours, sir," Jeffrey said. "Early Los Angeles class. Unimproved 688, pre-751 hull number."
"One of the flight-one boats still in commission," Wilson said. "Make that past tense." Jeffrey nodded. "It's not a lengthy list, Captain, who she could be … Sir, the crew may not have had time to destroy the crypto gear, and it probably wasn't all cremated by the atmosphere compression. Maybe we should try to find the stuff, before the enemy does."
"No, XO," Wilson said. He frowned. "We'll have to take the chance that this engagement was a double kill. We'll report it later. We don't have time or the right tools to do a proper inside search here."
For a while no one spoke. Jeffrey moved the probe along the side of the middle hull section, knowing this chunk would have been unoccupied.
"Radiation?" Wilson said.
"Just normal background," Jeffrey said, "when you account for the added seawater shielding. I'm getting a minor thermal plume, that's all."
"Good," Wilson said, "the reactor's stable. That core should be clean too. This boat would've been refueled recently."
"And no hull implosion," Jeffrey said. "It must have flooded first, before they went through crush depth."
"Switch back to passive," Wilson said. "Move to the next masscon contact, the one to starboard. Watch out for wreckage — there'll be
a nasty debris field out there."
"Understood." Jeffrey worked the trackball and his keyboard, once more seeing through the LMRS's eyes by the faint glow of bioluminescence, strobed by natural flashes and sheet lightning. A fish darted past, then another, too quick to make out details. Jeffrey guided the probe around a shadowy pile of mangled metal.
"Auxiliary machinery?" Wilson said.
"I can't tell," Jeffrey said. "It made quite a crater."
"Keep going."
"I'm at the second contact now."
"Okay, switch back to active laser line scan."
"Looks like the forward hull," Jeffrey said. "Probe's over the accommodation spaces, sir." For another long moment no one spoke.
"The hull's intact," Jeffrey finally said, to break the silence.
"Move back and forth," Wilson said. "I want to know what sank her."
"Oh boy," Jeffrey said when he found what he'd been looking for. "Here's the forward escape trunk, sir. Busted in completely"
"Not solely from blast shock," Wilson said. "The rim's too whole. That was water pressure, rupturing the weakened welds. Trunk gave out before the hull, catastrophic flooding. Again, no implosion."
Jeffrey saw more tortured pipes and cables down inside the trunk. "I'm moving on." Soon he reached the dead sub's bow
"Smashed in," he said. "Looks like she hit nose-first."
"This part did anyway," Wilson said as the probe explored the chaotic flattened tangle. " She may have broken up on the way down, from the strain, tumbling at over a hundred knots terminal velocity."
"Yes," Jeffrey said. He studied the image-enhanced display. "Look at how the ballast tanks accordioned … I see fragments of the sonar sphere."
"What's that thing off to the left?"
Jeffrey moved in closer. "It's a body, sir." Wormlike creatures swarmed around it.
"Christ," someone said.
"Easy, people," Jeffrey said. He took a breath. "There'll be more, inside or outside. We have to do this."
Wilson sighed. "XO, go to the engineering plant now."
Jeffrey piloted the probe, being careful not to snag the fiber-optic line. He stayed well off the bottom, afraid of what he'd see there. For a while the screen showed little, then the after portion of the hull came dimly into view.
