They continued north, into lands even more violently disarranged than those they had left behind. This was volcanic country, where the harsh sublimity of the southern highland was augmented by the ancient craggy peaks of Australis Tholus and Amphitrites Patera. The two volcanoes bracketed a region of lava flows, where the blackish rock of the land was frozen in weird lumps, waves, and rivers. Once these flows had poured over the surface in streams of white-hot fluid, and even now, hard and black and shattered by the ages, and covered with dust and ice flowers, the liquid origins were completely evident.

The most prominent of these lava remnants were long low ridges, like dragon tails now fossilized to solid black rock. These ridges snaked across the land for many kilometers, often disappearing over the horizon in both directions, forcing the travelers to make long detours. These dorsa were ancient lava channels; the rock they were made of had proved harder than the countryside they had originally flowed over, and in the eons since, the countryside had been worn away, leaving the black mounds lying on the surface somewhat like the fallen elevator cable only very much larger.

One of the dorsa, in the Dorsa Brevia region, had recently been turned into a hidden sanctuary. So Nadia drove their rover on a tortuous path through outlying lava ridges, and then into a capacious garage in the side of the largest black mound they had seen. They got out of their car, and were greeted by a small group of friendly strangers, several of whom Jackie had met before. There was no indication in the garage that the chamber beyond it was going to be any different from any other they had visited, and so when they walked into a big cylindrical lock and out the other door, it was a great shock to find before them an open space that clearly occupied the whole interior of the ridge. The ridge was hollow; the empty space inside it was roughly cylindrical, a tube perhaps two hundred meters floor to ceiling, three hundred meters wall to wall, and extending for as far as they could see in both directions. Art’s mouth was like a cross-section model of the tunnel: “Wow!” he kept exclaiming. “Wow, look at this! Wow!”

Quite a few dorsa were hollow, their hosts told them. Lava tunnels. There were many of them on Terra, but the usual two-magnitude scale jump obtained, and this tube was in fact a hundred times bigger than the biggest Terran tube. When the lava streams had flowed, a young woman named Ariadne explained to Art, they had cooled and hardened at their edges, and then on their surfaces— after which hot lava had continued to run through the sleeve, until the flows had stopped, and the remaining lava had emptied out onto some lake of fire, leaving behind cylindrical caves that were sometimes fifty kilometers long.