Mannion went to the phones to call the boat's chief engineer.

"Any idea what it is, Jonesy?" Mancuso turned back.

The sonarman shook his head.  "It isn't screw sounds.  I've never heard anything like it."  He ran the tape back and played it again.

Two minutes later, Lieutenant Earl Butler came into the attack center.  "You rang, Skipper?"

"Listen to this, Earl."  Mancuso rewound the tape and played it a third time.

Butler was a graduate of the University of Texas and every school the navy had for submarines and their engine systems.  "What's that supposed to be?"

"Jonesy says it's a Russian sub.  I think he's right."

"Tell me about the tape," Butler said to Jones.

"Sir, it's speeded up ten times, and I washed it through the BC-10 five times.  At normal speed it doesn't sound like much of anything."  With uncharacteristic modesty, Jones did not point out that it had sounded like something to him.

"Some sort of harmonic?  I mean, if it was a propeller, it'd have to be a hundred feet across, and we'd be hearing one blade at a time.  The regular interval suggests some sort of harmonic."  Butler's face screwed up.  "But a harmonic what?"

"Whatever it was, it was headed right here."  Mancuso tapped Thor's Twins with his pencil.

"That makes him a Russian, all right," Butler agreed.  "Then they're using something new.  Again."

"Mr. Butler's right," Jones said.  "It does sound like a harmonic rumble.  The other funny thing is, well, there was this background noise, kinda like water going through a pipe.  I don't know, it didn't pick up on this.  I guess the computer filtered it off.  It was real faint to start with—anyway, that's outside my field."

"That's all right.  You've done enough for one day.  How do you feel?" Mancuso asked.

" A little tired, Skipper.  I've been working on this for a while."

"If we get close to this guy again, you think you can track him down?"  Mancuso knew the answer.

"You bet, Cap'n!  Now that we know what to listen for, you bet I'll bag the sucker!"

Mancuso looked at the chart table.  "Okay, if he was heading for the Twins, and then ran the route at, say twenty-eight or thirty knots, and then settled down to his base course and speed of about ten or so . . . that puts him about here now.  Long ways off.  Now, if we run at top speed—forty-eight hours will put us here, and that'll put us in front of him.  Pat?"

"That's about right, sir," Lieutenant Mannion concurred.  "You're figuring he ran the route at full speed, then settled down—makes sense.  He wouldn't need the quiet drive in that damned maze.  It gives him a free shot for four or five hundred miles, so why not uncrank his engines?  That's what I'd do."