by Z. Albert Bell

XII. Compound Interest

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Within one of the legs of the thing that slowly crawled through dirt and stone was a passage only a few millimeters across. The leg was tightly packed with modules forming a dizzyingly interconnected system, some connected by tubes, others by black threads of powerful carbon fibers, and still others by electromagnetic signals or mechanical linkages.

The narrow gap that wove through the interconnected system was dark and silent. Through it, an inch-long, six-legged form doggedly climbed. It did not mind the dark, the silence, or that it was climbing upside down.

Externally at least, its body plan was largely unchanged from the cockroach that had inspired its design. Internally, however, it was somewhat different.

It no longer excreted pheromones the way its ancestors had. Its biology could still emit a variety of chemical signals and compounds, but for very different purposes.

It was different in other ways that would not have been obvious on visual inspection. The little creature was over two hundred years old. That was because it had better hygiene and medical care than any insect alive.

The maintenance drone shaped like a bug slowed as it reached a juncture. It turned and pulled itself into a side passage where it arrived at a closed seal. It released a chemical sequence from its body. Nearby biochemical sensors detected the signal and the seal opened, allowing the drone to crawl inside.

The chamber's curved walls weren't much farther apart than the walls of the maintenance passage. The drone waved feelers around the chamber, confirming that it had been fully drained of the high-energy-density fuel it was built to contain.

The fuel cell was one of many scattered throughout the excavator. This fuel would not be burned directly to power any systems; incredible amounts of potential energy were kept in forms suitable to long-term storage, which then would be converted to the specific forms most efficient for the excavator's disparate systems.

Satisfied with its inspection of the reservoir's walls, the drone approached the shape of a miniaturized pump. With deft motions, it cleaned the pump's internal and external surfaces, making note of its state of wear and cleaning away any accumulated gunk.

Partway through the routine task, the drone stiffened and turned to scuttle away at top speed as its internal state registered Emergency!

The drone's body had been given a radio antenna coiled up where its ancestors' wings would have been. Radio waves had stimulated a neural network that had been designed to recognize a limited vocabulary of radio signals.

Directives from the excavator's operator would arrive via modulated radio frequency, and the thing would scuttle to obey, easily slipping through the almost-nonexistent maintenance slots to perform its tasks. If the transmission was garbled or indistinguishable, the drone was built to assume the worst and return to its storage alcove for further instructions.

It did so now. After some time in darkness and silent stillness, it received a clear transmission ordering it to complete its original task.

The drone raced back toward the drained reservoir and resumed the interrupted operation. With preternaturally-dextrous claws and mouthparts, it disassembled and reassembled modular components, regurgitating solvents and epoxies in turn as it deconstructed and reconstructed machinery in miniature.

Before the last pieces could be locked in place, the drone received another unintelligible transmission, and in keeping with its instructions, raced back to its storage alcove.

When it got to the storage alcove it had returned to so many times before, it received a different reception this time. The drone found itself suddenly pulled out of its alcove and into a writhing pocket of corrosive fluids.

Many tiny delicate fingers reached into the caustic liquid and pulled the drone apart, setting aside reusable components and recovering a tiny memory storage unit. The automated process overseeing the drone would analyze its recorded experiences for anomalies.

Another drone took the place of its hapless predecessor, hurrying to the twice-abandoned fuel chamber to complete the deferred task. This drone did not sense any unusual transmissions as it carried out its new responsibilities.

Not right away.

---

The seas raged above, the mantle of the planet roiled beneath, and the tectonic plate in which I was embedded scraped by its neighbors noisily as I pushed my perception of time. Time slipped by faster and faster until--

I snapped back into a slower timeframe with resignation. What was it? Had I failed to account for a geological or chemical process? Had this world's life somehow managed to invade my excavator despite the pains I had taken?

The answer was neither, it seemed. I had designed my maintenance drones to use a low-energy radio signal for coordination of maintenance tasks, and that design had operated well for almost a quarter of a galactic spin as I had impatiently waited to unlock the first node in the payload. Several generations of them had been automatically recycled by automated processes under the assumption the failures were the result of manufacturing defects or the effects of wear and tear from operating on such long timescales.

That wasn't what had happened, though. The failures had continued at the same rate regardless of the redeployment of multiple generations of the drones.

I had designed the drone command system to use extremely low-power signals. Under this world's atmosphere, within its powerful magnetic field, underneath the deepest parts of the ocean and more often than not under a great deal of rock as well, I had not thought it necessary to provide my maintenance drone control system with electromagnetic shielding. The cosmic cacophony of radiation and echoes would not penetrate air, water, and stone, after all. I was well insulated from solar flares and the like.

But something had interfered with my maintenance drones. Strange. I checked in with the sentinel outposts I had established on the seabed and received negative responses when inquiring about any new arrivals in the system.

It must be some novel geological process, I thought. Natural fission reactors formed, if rarely; perhaps mineral deposits near one were acting as a natural capacitor and sending the occasional burst whose harmonics occasionally interfered with the sensitive radio organs of my drones.

I made a note to investigate the phenomenon later. I was so close to completing the first part of the payload. I would be able to target multiple nodes at a time after cracking the first one.

I absently modified the drones' control systems to account for occasional interference and configured the automated systems to catalog and analyze further information about the strange signals.

Almost there. I pushed time forward again, the remaining hashes began to drop, and--

BOOM.

I heard the detonation ring through the crust of the planet. I pushed the excavator up onto the seabed and rapidly checked again with the sentinels. Still no sign of the traitor. Another asteroid strike?

BOOM.

Another explosion shook the entire world. My mind raced in disbelief. Where were these explosions coming from? They were completely inconsistent with the seismic makeup of the planet.

This wasn't the release of long-pent-up potential energy of a volcano or an earthquake. Besides, those things became less frequent after the pressure was released; these explosions continued with a steadily building tempo.

BOOM.

I slowed my perception of time further still, struggling to grasp what was happening. None of the detonations had taken place under the sea. They all took place above, on the dry land I had relinquished after the extinction event.

Deep within the excavator, I let the octopoid form I still wore thrash in frustration. I needed to focus on recovering the Library. Too much time had passed already.

BOOM.

The vibrations rocked the planet again, from a completely different direction this time. This was not geological. This was not meteorological.

BOOM.

No. It couldn't be something that had evolved. I had accepted what I had seen so far in confused fascination, but I could not imagine any biochemical evolutionary pathway that could even begin to make this much noise in my seismological sense.

BOOM.

I came to the only conclusion I could: anything that could make that much noise for reasons I didn't understand-- and couldn't even hypothesize about-- was a threat. I slowed my perception of time still further, close to the current limit of my perceptual time resolution, and shook my mobile base of operations loose from the rock and coral of the sea floor. My excavator began to lumber its way toward the nearest landmass that had hosted the explosions. I needed to know what they were.

Ping.

I stopped the excavator in shock. That was a sonar ping. The last species I had studied that attempted echolocation had only used a very weak form of that sensory modality. In its original form it was only useful for obtaining prey, and then only because of that species' individuals' learned behaviors.

This sonar ping, however, was powerful. I sent back an answering ping, getting a sense for the terrain of the seafloor, and moved my excavator toward the source of the original ping at full speed.

I desperately needed to obtain specimens of the latest adaptations this world's life had evolved. Something had changed. Something had changed dramatically.

The bulk of the excavator launched itself over a ridge. I was using up my stores of energy at several hundred times the ordinary draw of the excavator, but I continued to burn through the stores as I surged toward a shape that was bigger than any life form I had ever seen on this planet.

I had restrained my curiosity for the good of my people, but now my ignorance might be the death of us all.

Within the body of the excavator, my miniature factories desperately built with the designs I had available. The excavator couldn't swim, but if I could get close enough, I wouldn't need to.

The shape grew larger in the water as I approached. The surface of the bizarre thing was opaque to infrared and sonar. It made strange noises in the water, humming and occasionally releasing its sonar ping. It did not move as I approached.

I readied the charges my constructors had fabricated, positioning the excavator and its limbs carefully as the sound of the sonar chirps increased in frequency and volume.

To my shock, light almost as bright as this world's primary star suddenly illuminated the darkness. What was this? Some kind of adaptation to blind predators or disorient prey? How was it so bright?

I detonated the first charges directly under my vessel and the excavator surged forward explosively. The legs of my craft twisted around and grasped at the enormous shape, locking against the surface of the creature and sinking digging claws into it.

Its armored exterior was stronger than any other living thing I had encountered. It was stronger than stone. This thing's exterior skin was made of... steel?

I prepared to tear into its interior and rip out its maddeningly unlikely innards for analysis. Answers. Then, I paused.

Pressurized gas was escaping where my claws had sunk into the side of the thing. The radio communications I had detected-- many more of them erupted from the creature, very powerfully. Subsystems throughout the excavator spasmed in shock as interference they had never been built to resist was released into them at point blank range.

Two legs of the excavator swung out and away as systems simultaneously threw errors or initiated incorrect procedures. The weight of the two malfunctioning legs overbalanced my entire craft, and it lost its grip on the enormous metal-skinned thing, falling away.

The large shape made a terrible sound and rocketed away at speeds unlike any other living thing I had seen. Moments later, explosions rocked the sea bed around the excavator and the waters were filled with choking dust and debris, although the excavator was not seriously damaged.

I reviewed the mounting failure notifications in disbelief. What had happened?

What WERE this world's life forms?

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