'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.
