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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.