BOUNDARY(41)

"Excellent. Down we go!"



Ariel could also see the same thing, and despite being orders of magnitude less intelligent than her master, quickly reached the same conclusion. The little probe paused, rotated, and descended into the abyss.



Fifty meters down.



With a slow and steady precision, Ariel passed by gray-black rock with occasional tinges of other colors like reddish-brown. The crack descended at a slant, and its irregular walls showed that some sort of violence had caused its opening. That wasn't much of a surprise, of course.



One hundred meters down.



Ariel slowed and rotated, seeing a large rock blocking part of the crack dead ahead. It was clear to either side, however, so the automated sensing drone continued its descent, having chosen the left-hand side of the rock to pass by.



Two hundred thirteen meters down.



Dark shadows showed on either side of the crack, indicating another cavity or cavern. As Ariel drew level with this new intersection, it was clear that whatever cataclysm had caused this crevice to open had also caused it to cut straight across another long, slender cavern. Ariel hovered at the three-way intersection, consulting its own data to decide its course—further down, or into one of the two tunnellike cavern segments. A.J. did the same, in case he had to transmit to Ariel to abort and take a different route.



But Ariel made the same decision he would have. The flow of water vapor outgassing was stronger from one branch of the bisected cavern.



Here, Ariel had a bit more room to maneuver. The tunnellike cavern, oval in cross-section, was ten feet wide and eight feet high. Strange rippled formations were visible along the walls at regular intervals. A.J. was reminded of the scalloping he had seen in several caves, but clearly these odd shapes could not have resulted from running water over millennia. He wondered if cometary outgassing could have a similar effect.



Two hundred meters farther along, the tunnel reached a branching. One side turned deeper into Phobos, while the other stayed roughly level beneath the surface.



Tinkerbell was starting to show a clear drop in signal strength from its sister drone, but both A.J. and Ariel calculated that at least another three hundred meters would be possible before it would be time to decide on whether Ariel would have to go totally solo or not. Once more Ariel selected the path with the strongest H2O concentration—this time continuing along the original passageway.



Two hundred meters more, and the signal was starting to show some signs of interference. Suddenly the walls fell away on three sides, leaving only one—the side towards the surface—relatively level with respect to the prior tunnel. On Earth, this would be the equivalent of entering a large room in a cavern, with the relatively level side being the "floor." Ariel began a surveying drift around the dim, airless space, which seemed to be huge, something like a football field across. Its light began picking up other cavern exits along the "floor" area.



A.J. realized how stiff his neck was. He'd been leaning forward, watching tensely, despite the fact that on a VRD leaning forward was just useless. Amazing what instinctive reactions will do. For what must have been . . .



Two hours? It was heading for six in the morning! He got up, stretched hugely, and slugged down the rest of his now-warm soda.



Something nagged at the corner of his vision, which had been concentrated on the mundane for the past couple of minutes. He refocused on his VRD screen and sat down. It had seemed that there'd been a flicker of slightly different color . . .



Ariel had noticed it too, apparently. The probe had many subroutines for analyzing images, noticing anomalies, and returning to them. It was slowing and turning around now. Using gyros, it spun in place. Something flicked across the field of view, then stopped and was centered. Slowly it began to grow, with slight flickers of interference across it, as Ariel approached.



A.J. was barely aware of himself standing slowly, his mouth half-open, hand stretching towards the virtual screen, his hindbrain trying to reach out and touch the image glowing before him.



Inset into the wall, perhaps a meter from the nominal "floor," was a massive...



Something. It shone with a brownish-gold luster, undeniably metallic. It was symmetrical, a generally triangular shape with rounded sides and smaller, round-sided triangles at each corner. Across its surface, in curved sequences like waves, mysterious black and silver symbols marched in organized ranks.



Ariel had stopped with the object just filling its field of view. This target lay entirely out of its search parameters, and it was now waiting to be told what to do.



"Well," said A.J. to the empty air around him. "That's something you don't see every day."