"We're not done yet." She slipped a jewel case from her tool kit, opened it, and took out a CD. "This is a bootable disc with an assortment of diagnostic tools. Let's take a closer look at Asher's hard drive."

She slipped the disc into the laptop and restarted it. This time the screen came to life. The disc drive trundled for a moment, then several windows opened. Hui took a seat at the lab table and began typing. Crane peered over her shoulder.

For several minutes, Hui moused her way through a variety of windows. Long series of binary and hexadecimal numbers appeared, scrolled up the screen, then disappeared again. At last, she sat back.

"The hard drive is operable," she said. "I can't detect any further physical damage."

"Then why can't we read it?" Crane asked.

Hui looked at him. "Because it appears somebody has erased all the data on it."

"Erased?"

She removed her face mask, shook out her hair, and nodded. "Based on the electromagnetic pattern, it seems somebody used a degausser on it."

"And this was done after the fire?"

"Must have been. Asher wouldn't have done it himself."

"But why?" Crane felt stunned. "That makes no sense. For all anybody knew, the laptop was ruined."

"I guess somebody wanted to make sure of that."

Slowly, Crane pulled out the other lab stool and sat down. He took off his own mask and dropped it on the table. All of a sudden, he felt very old.

"That's it, then," he said. "Now we'll never know what Asher found."

He sighed. Then he glanced at Hui. What he saw surprised him. She was looking back, a small smile on her face that-at any other time-he might have termed mischievous.

"What is it?" he asked.

"I still have one or two tricks up my sleeve."

"What are you talking about? The hard drive's been erased."

"Yes. But that doesn't mean the data's gone."

He shook his head. "I don't understand."

"It's like this. When you erase data on a hard drive, you're really just overwriting that data with random zeros and ones. But you see, when the read/write head writes that new data, it uses only enough signal necessary to set the bit. That's the way hard drives work: just enough signal, and no more."

"Why is that?"

"To make sure that adjoining bits aren't affected. Anyway, because the signal isn't powerful enough to fully saturate the platter, whatever data was previously there is-like a ghost-going to affect the overall strength of the signal in that location."

Crane looked at her, uncomprehending.

"Let's say you have two positions on a hard drive, side by side. The first contains a zero, the second contains a one. Then somebody comes along and overwrites those two positions with two ones. So now we have a one in both positions. But guess what? Because the read/write head uses the bare minimum of signal to write those ones, the position that had a zero in it before has a weaker signal strength than the position that had a one in it."

"So the data that was there before affects the new data that overwrites it," Crane said.

"Exactly."

"And you've got a tool that can resurrect that old, overwritten data?"

Hui nodded. "It takes an absolute value of the signal and subtracts it from what's actually on the hard disk. That leaves us with a shadow image of what had been there before."

"I had no idea that was possible." Crane paused. "But wait a minute. The data wasn't overwritten. You said it was degaussed. Demagnetized. How can you restore that?"

"Whatever kind of degausser was used, it doesn't seem to have been very powerful: probably a hand-held model. Or maybe the person who did this didn't take into account that the platters of the hard disk have a small amount of shielding. Anyway, a light degaussing is the equivalent of overwriting the hard disk two, maybe three times. And my equipment has the capability of restoring data that's been overwritten twice as many times as that."

Crane could only shake his head.

"But the process is destructive. We'll only have one pass at it-and that means we'll need another hard drive to dump the reconstructed data onto. I trashed mine when I removed the PCB." She glanced at him. "Can I borrow yours?"

Crane smiled. "Seems we're going through laptops pretty fast. Sure, I'll get it now."

"I'll get the data recovery started." Hui pushed the magnifying glass aside and reached for her tool kit.

"You be safe." And Crane turned and quietly left the lab.

41

The man calling himself Wallace limped quietly through the maze of passages making up the science facilities of the refitted Storm King oil platform. He moved more quickly than usual: he had only now received a message in his quarters-a coded signal, transmitted via low-frequency radio-and he had to pass it on to the operative on Deep Storm immediately.

The Tub left in twenty minutes, bound for the ocean floor. If he hurried, he might just make it.

Reaching his office, he turned on the light, then shut and locked the door. The courier bag sat on his desk, ready to be delivered to the Recovery Chamber on the lowest level of the platform. He opened it, rummaged through its contents, then pulled out a CD hand-labeled RADIOGRAPHS 001136-001152.

Image files-precisely what he needed.

He inserted the CD into his computer, loaded one of the images at random into memory. He removed the CD, placed it back in its jewel case, and returned it to the courier bag. Next, he wrote a short routine that would embed a message into the least significant bits of the pixels of the radiograph image. It was the work of five minutes to type in the computer program and double-check it for bugs.

The man pressed a function key, causing the short routine to execute. A question mark appeared on screen: the routine was requesting input. He carefully typed in the message he'd been ordered to relay. Then he paused, finger hovering over the enter key, while he examined the message for accuracy:

Satisfied, he pressed the key. The message disappeared from the screen; there was a brief pause as the program converted the message into its binary equivalent, then hid it within the digital code of the radiograph. A short chirp indicated that the process had been completed successfully.

Wallace smiled.

Opening a drawer, he pulled out a writable CD, slid it into the drive, and instructed the computer to burn a copy of the doctored radiograph. While the machine worked, he sat back in his chair, cleaning his glasses with his shirttail. The image was not large, and within a few minutes the new disk had been burned. He ejected it and quickly powered down the computer, instantly destroying all traces of his work. Pulling a fresh jewel case from the open drawer, he inserted the CD, wrote the recipient's name on it with a black marker, then slipped it into the courier bag.

He stood up, slung the bag over his shoulder, glanced at his watch.

Twelve minutes to spare. Excellent.

Unlocking his door, he stepped out, whistling to himself as he made his way to the Recovery Chamber and the waiting Tub.