The floor of this particular tunnel was approximately flat, and now it was covered by rooftops and grassy parks, ponds, and hundreds of young trees, planted in groves of mixed bamboo and pine. Long cracks in the roof of the tunnel had served as the basis for filtered skylights, made of layered materials which gave off the same visual and thermal signals as the rest of the ridge, but let into the tunnel long curtains of sunny brown air, so that even the dimmest sections of the tunnel were only as dim as a cloudy day.

Dorsa Brevia’s tunnel was forty kilometers long, Ariadne informed them as they walked down a staircase, although there were places where the roof had caved in, or plugs of lava almost filled the cavity. “We haven’t closed off the whole thing, of course. It’s more than we need, and more than we could keep warm and pumped anyway. But we’ve closed off about twelve kilometers now, in kilometer-long segments, with tent-fabric bulkheads between them.”

“Wow,” Art said again. Nirgal felt just as impressed, and Nadia was clearly delighted. Even Vishniac was nothing compared to this.

Jackie was already near the bottom of the long staircase that led from the garage lock to a park below them. As they followed her Art said, “Every colony you’ve taken me to I’ve figured has to be the biggest one, and I’m always wrong. Why don’t you just tell me now if the next one is going to be like all of Hellas Basin or something.”

Nadia laughed. “This is the biggest one I know of. Bigger!”

“So why do you all stay in Gamete, when it’s so cold and small and dim? Couldn’t the people from all the sanctuaries fit into this space?”

“We don’t want to all be in one place,” she replied. “As for this one, it wasn’t even here a few years ago.”

Down on the floor of the tunnel they appeared to be in a forest, under a black stone sky rent by long jagged bright cracks. The four travelers followed a group of their hosts to a complex of buildings with thin wooden walls and steep roofs upturned at the corners. In one of these they were introduced to a group of elderly women and men in colorful baggy clothing, and invited to share a meal.