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It was, however, increasingly disappointing. The more the returns came in and were processed, the more clearly the base on this side was delineated. And the less and less likely it became that there were, or ever had been, any entrances—natural or Bemmius-made—in this area.



Finally he shook his head. "Ladies, give it up. There's nothing to find here."



"Damn," said Madeline without rancor. "I was hoping we could find something fairly quickly—because I certainly don't like the idea of trying to dig our way down to the thing. Well, we still have more perimeter to check. Maybe tomorrow."



She started making her way down the backside of the ridge, which sloped much less steeply on that side.



It wasn't until a few moments later that it registered on A.J. that Helen hadn't responded. For a moment he almost panicked, until he saw that she was at the top of the ridge, looking outward. Maybe just admiring the view. It'd be a while—a long while—before any of them started taking that view for granted. Compared to Valles Marineris, the Grand Canyon in Arizona was a ditch.



"Helen, you okay?"



"Hmmm?" Her voice sounded distracted. He could almost see her expression; it was the one she wore when she almost had a problem solved. "A.J., what do your GPR scans say about the geology?"



"Geology? Well, I'm not an expert, but basically, um, we've got the top layer of crap, something that I guess might be sorta-sandstone under the dust and debris, then a bunch of denser stone, and something else under that. The sorta-sandstone isn't very thick, the denser stone is thicker—maybe fifty to a hundred meters total in this area, though I get vague indications it tends to be even thicker off to the west. The denser stone makes it real hard to see through past that, and I have to be honest that I wasn't using stuff that would look down past twenty to fifty meters anyway. Any tunnels we're looking for have to come nearer the surface than that. Why?"



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."