For almost the entire length, a huge, sweeping wall of smooth, black metal reared up from the floor and curved up and over, out of sight above their heads, to be lost below the roof of the cavern. It was immense—just a part of something vast and cylindrical, lying on its side, the whole of which must have stretched far down into the ice below floor level. At the near end, outside the control room, a massive, curving wing flared out of the cylinder and spanned the cavern above their heads like a bridge, before disappearing into the ice high on the far left. At intervals along the base of the wall, where metal and ice met, a series of holes six feet or so across marked the ends of the network of pilot tunnels that had been driven all around and over and under the object.

It was far larger than a Vega. How long it had lain there, entombed beneath the timeless ice sheets of Ganymede, nobody knew. But the computations of field-vector resultants collected from the satellites had been right; there certainly had been something big down here—and it hadn't been just ore deposits.

"Ma-an," breathed Stanislow, after staring for a long time. "So that's it, huh?"

"That is big!" Peters added with a whistle. The aides echoed the sentiments dutifully.

Stanislow turned to Mills. "Ready for the big moment, then, Captain?"

"Yes, sir," Mills confirmed. He indicated a point about two hundred feet away where a group of figures was gathered close to the wall of the hull, surrounded by an assortment of equipment. Beside them a rectangular section of the skin about eight feet square had been cut away. "First entry point will be there—approximately amidships. The outer hull is double layered; both layers have been penetrated. Inside in an inner hull . . ." For the benefit of the visitors, he gestured toward a display positioned near the observation window showing the aperture in close-up. "Preliminary drilling shows that it's a single layer. The valves that you can see projecting from the inner hull were inserted to allow samples of the internal atmosphere to be taken before opening it up. Also, the cavity behind the access point has been argon-flooded."

Mills turned to Cameron before going on to describe further details of the operation. "Lieutenant, carry out a final check of communications links, please."

"Aye, aye, sir." Cameron walked back to the supervisory console at the end of the room and scanned the array of screens.

"Ice Hole to Subway. Come in, please."

The face of Commander Stracey, directing activities out near the hull, moved into view, encased in its helmet. "All checks completed and go," he reported. "Standing by, ready to proceed."

"Ice Hole to Pithead. Report transmission quality."

"All clear, vision and audio," responded the duty controller from the dome far above them.