The tall man from Earth and his short companion from Mars faced that darkness and marched forward into the bowels of Mercury.

In the radiance of their suit-lights, Bigman looked curiously about at the tunnel, which resembled those he had seen on the Moon. Rounded out smoothly by the use of blasters and disintegrating procedures, it stretched out straight and even. The walls were curved and merged into the rocky ceiling. The oval cross section, slightly flattened above and quite flattened below, made for the greatest structural strength.

Bigman could hear his own steps through the air in his suit. He could sense Lucky's steps only as a small shock of vibration along rock. It was not quite sound, but to a person who had passed as much of his life in vacuum and near-vacuum as had Bigman it was almost as meaningful. He could "hear" the vibration of solid material much as ordinary Earthmen could hear the vibration of air which is called "sound."

Periodically they passed columns of rock which had been left unblasted and which served as buttresses for the layers of rock between the tunnel and the surface. Again this was like the mines on the Moon, except that the buttresses were both thicker and more numerous here, which was reasonable, since Mercury's gravity, small as it was, was two and a half times that of the Moon.

Tunnels branched off the main shaft along which they traveled. Lucky, who seemed in no hurry, paused at each opening in order to compare matters with the chart he carried.

To Bigman, the most melancholy aspect of the mines was the vestiges of one-time human occupancy: the bolts where illumo-plates must once have been attached to keep the corridors blazing with the light of day, the faint markings where paramagnetic relays must once have afforded traction for ore cars, occasional side pockets where rooms or laboratories must have existed, where miners might pause to eat at field kitchens or where samples of ore might be assayed.

All dismantled now, all torn down, only bare rock left.

But Bigman was not the man to brood overlong on such matters. Rather, he grew concerned at the lack of action. He had not come out here merely for the walk.

He said, "Lucky, the ergometer doesn't show a thing."

"I know, Bigman. Cover."

He said it quietly, with no special emphasis, but Bigman knew what it meant. He shoved his radio control to the particular notch which activated a shield for the carrier wave and scrambled the message. It was not regulation equipment on a space-suit, but it was routine for Lucky and Bigman. Bigman had added the scrambler to the radio controls when preparing the suits almost without giving the matter a conscious thought.

Bigman's heart was beating a little faster. When Lucky called for a tight, scrambled beam between the two of them, danger was near. Nearer, at any rate. He said, "What's up, Lucky?"

"It's time to talk." Lucky's voice had a faintly far-off sound, as though it was coming indeterminately from all directions. That was due to the inevitable lack of perfection of the part of the receiving unscrambler, which always left a small fraction of "noise."

Lucky said, "This is Tunnel 7a, according to the chart. It leads back by a fairly simple route to one of the vertical shafts leading to the surface. I'll be going there."

Bigman said, amazed, "You will? Why, Lucky?"

"To get to the surface," and Lucky laughed lightly. "Why else?"

"What for?"

"In order to travel along the surface to the hangar and the Shooting Starr. When I went to the ship last time, I took the new inso-suit with me."

Bigman chewed that over and said slowly, "Does that mean you'll be heading for Sun-side?"

"Right. I'll be heading for the big Sun. I can't get lost, at least, since I need only follow the coronal glow on the horizon. It makes it very simple."

"Come off it, Lucky, will you? I thought it was the mines that have the Sirians in them. Didn't you prove that at the banquet?"

"No, Bigman, I didn't prove it. I just fast-talked it into sounding as though it were proven."

"Then why didn't you say so to me?"

"Because we've argued this out before, I don't want to go into it. I can't risk your losing your temper at the wrong time. If I had told you our coming down here was part of a deeper plan and if, for any reason, Cook had irritated you, you might have blurted it right out."

"I would not, Lucky. It's just that you hate to say anything at all till you're all ready."

"There's that too," admitted Lucky. "Anyway, that's the situation. I wanted everyone to think I was going into the mines. I wanted everyone to think I hadn't the foggiest notion of heading for Sun-side. The safest way of seeing to it was to make sure nobody, but nobody, not even you, Bigman, thought any differently."

"Can you tell me why, Lucky? Or is that still all a big secret?"

"I can tell you this. I have a strong notion that someone at the Dome is behind the sabotage. I don't believe in the Sirian theory."

Bigman was disappointed. "You mean there's nothing down here in the mines?"

"I could be wrong. But I agree with Dr. Cook. It is just too unlikely that Sirius would put all the effort that would be involved in setting up a secret base on Mercury just to achieve a bit of sabotage. It would be much more likely that, if they wanted to do such a thing, they would bribe an Earthman to do it. After all, who slashed the inso-suit? That, at least, can't be blamed on Sirians. Even Dr. Peverale hasn't suggested there are Sirians inside the Dome."

"Then you're looking for a traitor, Lucky?"

"I'm looking for the saboteur. He may be a traitor in the pay of Sirius, or he may be working for reasons of his own. I hope the answer is on the Sun-side. And I hope, furthermore, that my smoke screen concerning an invasion of the mines will keep the guilty person from having time to cover up or from preparing an uncomfortable reception for me."

"What answer do you expect?"

"I'll know when I find it."

"Okay," said Bigman. "I'm sold, Lucky. On our way. Let's go."

"Hold on, there," cried Lucky in honest perturbation. "Great Galaxy, boy! I said I'm going. There's only one inso-suit. You'll stay here."

For the first time, the significance of the pronouns Lucky had used sank into Bigman's consciousness. Lucky had said "I," "I." Not once had he said "we." And yet Bigman, with the easy confidence of long association, had assumed that "I" meant "we."

"Lucky!" he cried, torn between outrage and dismay. "Why do I have to stay?"

"Because I want the men at the Dome to be sure that we're here. You keep the chart and follow the route we talked about or something like it. Report back to Cook every hour. Tell them where you are, what you see, tell the truth; you don't have to make anything up-except that you say I'm with you."

Bigman considered that. "Well, what if they want to talk to you?"

"Tell them I'm busy. Yell that you think you've just seen a Sirian. Say you've got to cut off. Make up something, but keep them thinking I'm here. See?"

"All right. Sands of Mars, you'll go to Sun-side and have all the fun, and I'll just wander around in the dark playing games on the suit radio."

"Cheer up, Bigman, there may be something in the mines. I'm not always right."

"I'll bet you are this time. There's nothing down here."

Lucky couldn't resist a joke. "There's the freezing death Cook spoke about. You could investigate that."

Bigman didn't see the humor. He said, "Aw, shut up."

There was a short pause. Then Lucky placed his hand on the other's shoulder. "All right, that wasn't funny, Bigman, and I'm sorry. Now cheer up, really. We'll be together again in no time. You know that."

Bigman pushed Lucky's arm to one side. "All right. Drop the soft soap. You say I've got to do it, so I'll do it. Only you'll probably get sunstroke without me there keeping an eye on you, you big ox."

Lucky laughed. "I'll try to be careful." He turned down tunnel 7a but had not taken two steps when Bigman called out.

"Lucky!"

Lucky stopped. "What?"

Bigman cleared his throat. "Listen. Don't take stupid chances, will you? I mean, I'm not going to be around to drag you out of trouble."

Lucky said, "Now you sound like Uncle Hector. Suppose you take some of your own advice, eh?"

It was as close as they ever got to expressing their real affection for one another. Lucky waved his hand and stood glimmering for a moment in Bigman's suit-light. Then he turned and went off.

Bigman looked after him, following his figure as it gradually melted into the surrounding shadows until it turned about a curve in the tunnel and was lost to him.

He felt the silence, and the loneliness doubled. If he had not been John Bigman Jones, he might have weakened with the sense of loss, been overwhelmed at finding himself alone.

But he was John Bigman Jones, and he set his jaw and clamped his teeth and marched farther down the main shaft with unshaken tread.

Bigman made his first call to the Dome fifteen minutes later. He was miserable.

How could he have believed that Lucky seriously expected adventure in the mines? Would Lucky have arranged to make radio calls for the Sirians to pick up and keep tabs on?

Sure, it was a tight beam, but the messages weren't scrambled, and no beam was so tight that it couldn't be tapped with patience.

He wondered why Cook allowed such an arrangement, and almost at once the thought occurred to him that Cook disbelieved in the Sirians too. Only Bigman had believed. Big-brain!

At the moment, he could have chewed through a spaceship hull.

He gathered in Cook and used the agreed-upon signal for all clear.

Cook's voice at once shot back. "All clear?"

"Sands of Mars! Yes. Lucky's up ahead twenty feet, but there's no sign of anything. Look, if I've buzzed all clear, take my word for it next time."

"Let me talk to Lucky Starr."

"What for?" Bigman kept it casual with an effort. "Get him next time."

Cook hesitated, then said, "All right."

Bigman nodded to, himself grimly. There'd be no next tune. He'd buzz all clear and that would be all… Only how long was he supposed to wander about in the darkness before he heard from Lucky? An hour? Two? Six? Suppose six hours went by and there was no word? How long should he stay? How long could he stay?