From ``Earth'', by David Brin.

A new plaque glittered in the sunshine.

She shook her head. ``Oh, they'll store germ plasm. And maybe someday, long after the last one has died . . .''

Her voice just faded then, and she looked away.

At the time Alex had only a vague notion what it was all about, what the ark program was for, or why the agencies involved had at last given up the fight to save the Indian forests. All he knew was that Jen was sad. He took his grandmother's hand and held it quietly until at last she sighed and turned to go.

These feelings lingered with him even long after he went away to university and entered physics. Everyone is either part of the problem or part of the solution, he had learned from her. Alex grew up determined to make a difference, a big difference.

And so he sought ways to produce cheap energy. Ways that would require no more digging or tearing or poisoning of land. Ways to give billions the electricity and hydrogen they insisted on having, but without cutting any more forests. Without adding poisons to the air.

Well, Alex reminded himself for the last time. I may have failed at that. I may have been useless. But at least I'm not the one who killed the Earth. Someone else did that. It was a strange, ineffective solace.

No, another part of him agreed. But the ones who did it -- whatever team or government or individual manufactured Beta -- they, too, might have begun with the purest of motives.

Their mistake might just as easily have been my own.

Alex remembered the tigress, her savage, reproachful eyes. The slow, remorseless pacing.

The hunger . . .

Now he pursued a far deadlier monster. But for some reason the image of the great cat would not leave him.

He remembered the blackbucks, gathered in their pen all facing the same way, seeking security and serenity in numbers, in doing everything alike. Tigers weren't like that. They had to be housed separately. Except under rare circumstances, they could not occupy the same space. That made them harder to maintain.

There were analogies in physics . . . the blackbucks were like those particles called bosons, which all sorted together. But fermions were loners like tigers. . . .

Alex shook his head. What a bizarre line of contemplation! Why was he thinking about this right now?

Well, there was that postcard from Jen . . .

Not really a postcard -- more a snapshot, sent to one of his secret mail drops in the Net. It showed his grandmother, apparently as spry as ever, posing with several black men and women and what looked like a tame rhinoceros -- if such a thing were possible. Transmission marks showed it had been sent from the pariah Confederacy of Southern Africa. So Jen was making waves, still.

It runs in the family, he thought, smiling ironically.

He jerked slightly as someone nudged him on the shoulder. Looking up, he saw George Hutton standing over him.

``All right, Lustig, I'm here. Stan tells me you wanted to show me something before we begin the next test run. He says you've added to your bestiary.''

Alex jerked, still remembering the life ark. ``I beg your pardon?''

``You know . . . black holes, microscopic cosmic strings, tuned strings . . .'' George rubbed his hands in mock anticipation. ``So, what have you come up with this time?''

``Well, I've been wrong before . . .''

``And you may be again. So? Each time you goof, it's brilliant! Come on, then. Show me the final loop, or lasso, or lariat, or . . .''

He trailed off, eyes widening at what Alex manifested in the holo tank. ``Bozhe moi,'' George sighed. An expression Alex knew was definitely not Maori.

``I call it a knot singularity,'' he replied. ``An apt name, don't you think?''

The blue thing did resemble a knot of sorts -- a Gordian monstrosity with the same relationship to a boy scout's clove hitch as a spaceship had to a firecracker. The writing orb was in ceaseless motion -- loops popping out of the surface and quickly receding again -- making Alex think of a ball of angry worms. All around the rippling sphere was emitted a shining light.

``I -- I suppose that thing is made of . . . strings?'' George asked, then swallowed.

Alex nodded. ``Good guess. And before you ask, yes, they're touching each other without reconnecting and dissipating. Think of a neutron, George. Neutrons can't exist for long outside an atom. But contained inside, say, a helium nucleus, they can last nearly forever.''

George nodded soberly. He pointed. ``Look at that!''

The loops popping out of the roiling mass mostly throbbed and flailed quickly before being drawn back in. Now though, a string extended farther out than usual and managed to cross over on itself beyond the knot.

In a flash it burned loose and floated away from the greater body. Released from the whole, the liberated loop soon twisted round itself again. With another flash of reconnection there were two small ones in its place. Then four. Soon, the rebel string had vanished in a rush of division and self-destruction.

As they watched, another loop cut itself off in the same way, drifting off to die. Then another. ``I think I see,'' George said. ``This thing, too, is doomed to destroy itself, like the micro black hole and the micro string.''

``Correct,'' Alex said. ``Just as a black hole is a gravitational singularity in one, a knot is a discontinuity in space-time that can twist in three, four . . . I haven't calculated how many directions it can be tied in. I can't even dream what the cosmological effects might be, if any truly big ones were made back at the beginning of the universe.

``What all three singularities have in common is this. It doesn't pay to be small. A small knot is just as unstable is a microstring or a microhole. It dissipates -- in this case by emitting little string loops which tear themselves apart in a blaze of energy.''

``So,'' George said. ``This is what you now think you made in your cavitron, in Peru?''

``Yes, it is.'' Alex shook his head, still unable to really believe it himself. And yet no other model so accurately explained the power readings back at Iquitos. None so well predicted the mass and trajectory they had observed during the last week. It still astonished Alex he could have constructed such a thing without it was even theoretically possible. But there it was.

Silence between the two men remained unbroken for moments.

``So now you have a model that works,'' George said at last. ``First you thought you had dropped a black hole into the Earth, then a tuned string. Now you call it a knot . . . and yet it still is harmless, dissipating.''

Hutton turned back to look at Alex again. ``That still doesn't help you explain Beta, does it? You still have no idea why the other monster is stable, self-contained, able to grow and feed at the Earth's core, do you?''