Lenny pumped the intakes, swelled the slush balloons, and slowed Gaius to a hover. Switching on every spider-mounted floodlamp and calling attitude-control jets into play, he swept the ship into a graceful banking maneuver, as if it were a helicopter, and aimed the prow at true magnetic north. From an altitude of forty meters, viewed through cameras approximating ordinary human eyesight, the pads would have shown only darkness below. Yet the spider’s deeppenetrating cameras revealed a lava lake passing beneath their feet.

The lava, like the hydrocarbon stew that surrounded Gaius, was transparent, but its temperature was so much higher that in every direction except straight down it reflected sonar and lamplight and microwave lasers like a silver-plated tray. Unable to contain herself, Sharon immediately christened the volcanic caldera Mirror Lake. From all indications it was spread over thousands of square kilometers.

Eastward, where a thin crust was trying to form, the lake lost its immobility. Giant rafts of ice shuddered and reeled, creaked and boomed through ten thousand vents. There were volcanic fountains on the eastern horizon, but no choking fumes, no steam. At twenty-two hundred atmospheres, it was impossible for bubbles to form without being pressed immediately out of existence.

And strangest of all was the substance of the caldera. The boulders at the volcano’s western rim, over which falls of lava half a kilometer high spilled and froze, were composed mostly of water. Here the sheer weight of overlaying nitrogen and gasoline produced new pressure phases, or densities, of ice. The rim of Mirror Lake was Ice Three, which differed from ordinary ice in having more closely packed molecules and a melting point of 34.7 degrees below zero.

More cryovolcanism, Vinny thought. Just like what we saw on Triton. But a water volcano at minus 34.7 degrees centigrade seemed as inexplicable to him as Triton’s nitrogen geysers, and for the same reason. Though far below freezing, minus 34.7 degrees was both figuratively and literally hot lava by the standards of the outer solar system. When he tried to visualize how a magma chamber could be kept warm at this distance from the Sun his eyes kept turning toward the sky, toward frictional heating caused by Triton’s chaotic orbit overhead. Though Triton whirled around the planet at nearly the same distance that the Moon circled Earth, Neptune’s seventeen Earth masses caused Triton to circle five times faster than Earth’s moon. It was also circling backward, east to west, in a direction opposite to Neptune’s rotation—which explained why, from the icefields beyond Mirror Lake’s rim, the ethane-methane ocean conducted to Gaius the sounds of the planet’s surface quivering and snapping. As near as Vinny could tell, a planetary shell of Ice Three was being continually pulled apart, straining to break into great crustal blocks. He imagined that at the equator the solid, Ice Three surface of Neptune was bobbing up and down, producing an otherworldly tide that could easily put the Bay of Fundy to shame. Quick mental calculations suggested that the current rate of internal heating due to tidal action was enough to sustain chambers of molten ice, or water, inside Neptime, if the interior had, during the past billion years or so, been melted by something else—perhaps, Vinny guessed, by Lenny’s vagrant star.