Boundary 1: Boundary (51 page)

She turned and gazed off towards the west, then back to the east. "Where were the close orbital photos of this area?" A moment later: "Yes, those are the ones. Thanks."

For a few more minutes there was silence. Then she said briskly, "We're looking in the wrong area. We want the east side."

"Over at Thoat Canyon, you mean?"

"Exactly. But it's not a canyon."

"Huh? Then what is it?"

Helen's voice was growing more excited and animated as she worked her way back down to his level. "I think it's the biggest damn sinkhole I've ever seen in my life. Look at the pictures again."

A.J. called up the images. The gash in the Martian landscape, now that he studied it more carefully, did have the folded, crumpled look of something that had collapsed from below. "But aren't most sinkholes round?"

"Generally, yes, but there are reasons it might not be. The way things sit in this region, and the shape of that collapsed area . . . What I think we have here is a collapse of a cavern which had a thin roof of volcanic basalt over it. The basalt is thicker in this area, but thinned drastically over there. That was at the edge of the flow, or some area that for some reason didn't get covered as well."

"A cavern . . .
thirty kilometers long
?"

"A network of them, maybe. Plus the gravity's so much less. Damn, I wish Ryu were still here. He'd know."

"Well, we can bounce this up to
Nike
and see what the experts have to say."

It wasn't long before Chad Baird, one of the planetographers, was on the radio with them.

"Quite possible. What you may have there is an area that used to have fossil ice in it. The ice slowly—over a period of millions of years—sublimed away and percolated through the rock above. It would refreeze in the upper layers, weakening them, then evaporate away again when things warmed up. Eventually there would be a huge empty space and weakened rock to give way during one of the infrequent major Martian quakes."

"And if the cap stayed intact here, and was thicker . . . Is it possible there could be some ice left under this area?"

"There could be, Helen, yes. With a large deposit underground, and a relatively impermeable basalt cap on top . . . Yes, there could be."

A.J. saw where this was going. "Good call, Helen. That's real good."

"You get it, then? Our best chance for any entrance is obviously on the wall of Thoat Canyon near the base, and the base is here almost certainly because somewhere down here they found water— which they needed even more than we do."

"Joe was right, then," A.J. mused. "It was no coincidence at all that
Pirate
wound up so close. That's one of the things we in Ares were looking for, too—water. And we decided to look here because Valles Marineris gets you deeper into the surface of Mars than anywhere else on the planet."

He was getting excited, now. "And if there is such an opening, I might be able to find it the same way we found the one on Phobos."

"Exactly. If there's any connection to fossil ice, any opening will show a higher water concentration."

A.J. found himself grinning in anticipation. "Let me get back to
Thoat
and do some work. We just might find our way in after all!"

Helen's heart pounded as she was slowly lowered down the side of the three-hundred-meter cliff. "How much farther?"

"About fifty meters," A.J. answered. "Don't worry, I'm already here. Me and Madeline will pull you in."

Her hunch had been right. A.J.'s sensors, concentrated in the area along the rim of Thoat Canyon for several hundred yards, had in a day or so pinpointed three sources of excess water vapor, one of them clearly larger than the others. Lowering Willis via
Thoat
's winch, A.J. had found an opening nearly three meters high and almost that wide.

Although well-suited for open ground exploration, Willis was not designed for a spelunking expedition. There was no way around it but that they would have to take the risks themselves.

The fact that it was her idea and that she was the closest thing they had left to a geologist did little to comfort Helen on the way down. A three-hundred-meter drop would be fatal even in Mars' thirty-eight percent gravity. It was possible her suit might save her from instant death, but that sudden stop at the bottom would almost certainly leave her in critical condition—and with no medical facilities around to speak of, that would be even worse than immediate death.

However, she made it without incident. The other two pulled her inside and tied off the cable to a rock a few feet into the cave. It was with considerable relief that she felt her boots once more on solid stone. "All right, let's see what we have."

"Let's get these spare oxygen tanks set up first," A.J. said. "If an emergency comes and we wind up needing them, we don't want to be fumbling around."

That took only a minute. "Okay. Helen, I think you and Madeline should lead the way. I'll be leaving a trail of bread crumbs."

A.J.'s "bread crumbs" were small sensor and radio relays, which would permit them to continue to talk to the others even if they got quite a ways underground. That was necessary, if for no other reason than that Ken Hathaway had invoked his captain's authority and flatly forbidden them to go underground unless they could maintain communications.

It was a good idea anyway, leaving communications aside. Helen had some experience with exploring caves, and it turned out Madeline had even more. (For the usual non-reason:
Spent five weeks, once, with a group of fanatic spelunkers. No, don't ask why. It's still classified.
) Both of them knew how easy it was to get disoriented underground, with no lights beyond what they brought with them. But if they did get lost, A.J.'s bread crumbs would guide them out.

The tall opening in reddish-black stone slanted sharply downwards after the first few meters. It did not change much in size for quite a while, however, as the little party continued following it. After about sixty meters, the tunnel mostly leveled out and continued deeper into the cliff, towards the base.

"Dropping first relay," A.J. stated.

The walls of the tunnel were rough, showing no sign of intelligent shaping that Helen could detect. That didn't necessarily mean anything, of course. It did a dogleg to the left and then curved towards the southeast.

"That isn't very promising," she remarked, as it clearly took them away from the base.

"Don't start complaining yet," A.J. said. "I'm still picking up water concentration."

"Full stop!" Madeline said from just ahead. Cautiously, they moved up at her signal to take a look.

The tunnel seemed to end here, until they got close and saw that what happened was that the tunnel itself had been essentially beheaded. A short bit of it remained ahead of them, while at their feet a wide crack dropped vertically into the depths. It was two meters across and much longer than that. From their current vantage point, they couldn't tell how much longer.

Madeline shone her light down the crack. There seemed to be a floor, over a hundred meters down. "Good enough, I think. Helen?"

The paleontologist agreed. "Yes, we'll make the descent. One at a time. I think you should go first, A. J., then me. Madeline, you bring up the rear since I think you're our best climber in case something goes wrong." She cocked her head. "Am I right?"

"Probably. I spent some time on the Matterhorn, once."

Seeing Helen's eyes roll in her helmet, Madeline protested: "Hey, it was just for the fun of it! I was on vacation, and I rather enjoy danger sports."

Now A.J. was rolling his eyes.

"It's true!" Madeline insisted. "The only top secret involved is how I managed to evade the attentions of Swiss males with too much testosterone and too little common sense. That's still classified. My boss is real big on contingency planning. If Switzerland ever drops her neutrality and becomes a U.S. ally, he thinks I might have to go in and establish the security for their chocolate recipes."

Chuckling, A.J. dropped another bread crumb and began to lay out the climbing gear, while Madeline and Helen judged where best to put the pitons.

The descent was made quickly. Getting back up would be a bit more tedious, but they had a powered ascender device that would be able to bring at least one person up to the top. From there, with muscles on top and bottom working against low gravity, they should have no problem getting the other two out.

The crack became much broader as they went down. Once they were at the bottom, they could see that it had gone from two meters across to at least thirty meters. The length was still impossible to determine.

There were no fewer than three tunnels exiting nearby. A.J. set his sensors in each one and after several minutes declared that he couldn't tell which way to go. The water vapor concentration in each was roughly equal, at least to the limit of what he could analyze at this point.

"Which way is the base?" Helen asked.

Seeing A.J. point, she nodded at the exit closest to that direction.

"Then let's try this one."

"Works for me."

Narrower than the others, this passage required them to go single file and watch their heads. It twisted and turned and A.J. used three more bread-crumb relays.

"How many of those do you have left?"

"Forty-five. Believe me, I came prepared, especially after my experiences with that 'block out everything but gravity' stuff on Phobos."

"It appears to be widening," Madeline said. Once again, she was in the lead. "I think I see a larger area in front of us."

They emerged into a cavern and shone their lights around. "Oh, my God," Helen breathed.

They were now probably three hundred meters down, below the floor of Thoat Canyon by a considerable distance. The cavern loomed above them, its ceiling nearly fifty meters above their heads and its sides disappearing into the darkness beyond the reach of their lights.

But it wasn't the impressive sides that caught their attention. It was the diamond-bright sparkle from above, where pure white shimmering crystals hung from every point, where long stalactites glittered like diamonds, where other surfaces were coated with a luxurious white fur.

"Ice," A.J. pronounced.

Helen found herself almost unable to speak for a moment. Partly because the faint worry that they might run out of water if the reactors malfunctioned had now vanished. But, mostly, just because the fairylandlike sparkle was visually stunning.

After a while, though, she began thinking of the puzzle involved, and its possible implications. "How could there be that much water here? In this atmosphere? Could any fossil deposit have lasted this long with direct openings to the outside?"

"Might not have to," Chad Baird's voice broke in. Apparently the scientist had been listening to their transmissions. "One of the major theories—still being debated, but it's got strong support—is that Mars actually has a water table. Clearly it used to have a huge amount of water, and the theory is that a lot of it remains underground, maybe a kilometer or two. If so, what you may have is one, two, or three different interacting layers of water transport depending on depth, temperature, pressure, and some other less important factors. In that case, what might be happening here is that the fossil ice sublimes, gets redeposited in the caverns you've reached, and is at least partially replenished by water vapor percolating up through the soil beneath. Remember, you're way down below the general level of the ground there in the Valles. So if there is a water table, you're a lot closer to it. And with what you're seeing, that sounds more likely than ever."

Helen nodded, still staring. "And if sublimation and redeposition is keeping these structures around in this volume, there must be a fair amount of water farther down here."

A.J.'s enthusiasm came to the fore. "Let's find that base!"

Across the cavern they walked, cautiously. After they found many signs of impacts, they kept a wary eye on the inverted faerie realm above, in case one of the ageless decorations decided to become a slow-motion plummeting missile.

It was Madeline who spotted the path that led upward in the direction of the base. It looked strange, as if it were made of melted wax or something similar. None of them could decide if it had been created by nature or intelligence.

Surprisingly, A.J. held out for natural causes. "We know there were aliens about so we tend to think everything weird is their doing. But I'm betting that Mars itself has got more weird saved up for us than Bemmie could have cooked up in the short time he was here. Maybe it's something left over from the era when Mars had lots of running water."

"Doubtful," Baird commented. "A lot of the rocks involved in water deposition are, themselves, fairly heavy with water content. They tend to disintegrate in this kind of atmosphere as they dehydrate. But you have basalt over the top of the area, so it's possible that you're looking at something caused by volcanic action eons ago."

The steep path, if that's what it was and not some kind of lava-flow or other natural channel, took considerable time and effort to climb. Finally, they stood at the top and looked down a tunnel about the same size as the last one. Helen eyed the ceiling suspiciously; there were signs of cracking at intervals along the way.

And then they rounded the corner and found the door.

The construction of the doorway was similar to those found in Phobos base, but clearly not identical. A.J. sent back high-resolution pictures in all spectra to
Thoat
. Inside the rover, Joe studied the doorway and other structures partially visible around the door before they disappeared into the rock at either side. He looked at them both in normal imagery and the more esoteric, partial imagery A.J.'s sensors could extract out of the stubbornly shielded material.

"It's not the same," he said finally, "but it looks like the same kind of technology was involved. That's the best way I can put it. It doesn't differ from the Phobos designs nearly as much as those do from ours. It's more like the difference between Egyptian and Greco-Roman architecture."