At this thought, a chill ran up his spine and over his scalp. Involuntarily, he turned around to look behind him. There was, of course, no group of Martians advancing silently towards him. He laughed.

'What's so funny?' Bronski said.

'Nothing in particular. I laughed because... it doesn't matter. Joy, maybe. Here. Get the kit out.'

He turned his back to Bronski, who removed a box from the cylinder on it. This was a minilaboratory designed for making chemical-physical tests. Bronski put the box on the ground, opened the lid, and he and Orme went through the process with a swiftness owing to long training. When they were done, Orme gave his report.

'The door looks like metal. As you heard through the audiometer, the interior is hollow. It rings when hit with a steel hammer. Even a diamond won't scratch it. Nitric acid leaves it unmarked. I don't want to use a laser beam on it because air might damage the contents. Providing there are any. Whatever material it's made of, it's unknown to Terrestrial science.'

Bronski replaced the box in the cylinder, and they walked back to the Barsoom. The cement was hard. In this atmosphere, where the pressure was equal to that ten miles above Earth's surface, the moisture quickly left the cement. It had boiled off in a vapour invisible in this twilight.

Orme used a tiny jack to draw up the slack and make the cable taut. Now even a 250-mile-an-hour wind, which wasn't likely at the canyon bottom, would not be able to push the lander over.

Nadir Shirazi, who was spelling Danton now, said, 'How do you two feel? Do you want to rest before you go to the tunnel?'

'I'm too excited to stop now,' Bronski said. 'I'd like to push on.'

From the compartment which had held the anchoring material, they removed a telescoping aluminium ladder and a box of explosives. Orme carrying the box, they walked to the edge of the tunnel. The rover followed them, its main scanner keeping them in view for the two in the Aries and the billions of people on Earth. Orme put the box down and opened its lid. Bronski lowered the ladder down into the tunnel. With a powerful lamp he'd taken from the box, Orme played a beam of light along the tunnel. At the left side of the two men, the rover followed the light with its antennas.

Orme had seen the interior of the opening many times by courtesy of the rover. But now that he was seeing it with his own eyes, he felt the same thrill as when he'd first witnessed it in the Houston laboratory. At the far end was a mass of rock, pieces of the fallen roof. These presumably covered another door. Along the length of the floor were other stone chunks, large and small. At the other end was the upper part of a door, its lower quarter behind more pieces of rock. Red dust covered the rocks. But its thinness indicated the roof had caved in recently.

What had caused the collapse? No one had a theory which could hold up under any rationalisation. The tunnel was too tar away from the nearest cliff for any rocks to have fallen on it. Anyway, there were no large rocks inside the tunnel or near it. To the west were some huge boulders, but these had been trundled down the canyon floor by water in some very remote past.

One scientist had proposed that a small meteorite had shattered the roof. But the area around here was free of any impact craters, small or large. And it seemed too much of a coincidence that a rare meteorite should happen to strike this very narrow area and reveal what would otherwise never have been discovered.

Orme steadied the beam on the orange characters in the dull black door. Tau Omega in majuscule writing. But had they been made by one versed in Greek? Were not the letters so simple that they would have been used by other sentients? T and . would naturally occur to anyone who was originating an alphabet. If indeed these characters were alphabetical. They could just as easily be letters in a syllabary, or in an ideogrammatic system such as the Chinese, used. They could also be arithmetical symbols.

Orme gestured to Bronski to go, down the ladder. If he wasn't to be the first man to step on to Mars, he at least could be the first to touch the door in the tunnel.

The rover was on the edge of the opening now, one scanner on Orme and the other following the Frenchman. When Orme saw that Bronski was off the ladder, he dropped the box to him. Bronski caught it easily, and Orme went down the ladder.

Bronski had climbed up the small pile of rocks and was examining the door by the time Orme reached him. Orme picked up a stone about the size of his head and heaved it up and out of the tunnel, first making sure that the rover wouldn't be struck by it. Bronski came down from the top of the pile to help him. In about five minutes the way to the door was cleared. In the light of the four-legged lamp, which had been set on the floor, Bronski removed the box from the cylinder on Orme's back. The tests revealed that the door was of a steel alloy.

'It's set within the opening very tightly,' Orme said. 'Obviously, it's an air-pressure seal, designed for just what happened, the collapse of a section of tunnel.'

Unlike the shell of the supposed spaceship, the door was thick. There was no hollow echo when he hit it with the hammer.

'We could try to blow it out,' Orme said. 'But I think it'll be easier of we go to the roof of the next section and dig down.'

They got out of the tunnel and returned to the lander. Orme was beginning to get tired, which meant that Bronski should be even more fatigued. Orme was only five feet eight, but he weighed a muscular 190 pounds on Earth, with no excess fat. The slender Bronski was quick, but he could not keep up with his captain.

Orme suggested that they eat while taking a rest and perhaps even grab a nap. The Frenchman refused.

'I'm still too keyed up.'

Carter, however, from his command post in Houston, ordered that they attach the monitors. After reading the indicators, he said, 'You guys will have to recharge your batteries. You're really tired.'

By the time the message came through, they had eaten. For an hour they rested in their reclined seats. Orme used alpha-wave techniques to get to sleep. Even so, it took twenty minutes, according to the monitors, before he succumbed. He would have sworn that he'd been awake the entire period.

Twenty minutes later, they were back at the tunnel site. Eighteen inches beyond the door, Orme cut a hole into the tunnel roof with a small laser-tipped drill. When it broke through, the explosion of enclosed air drove the tool up out of the hole. But Orme, expecting this, was standing to one side. Even so, the drill was almost jerked out of his hand.

He set to work at once to drill five more holes, all in a circle with a diameter of three feet. He could have connected the holes with the laser and cut a complete section to drop down into the tunnel. But he had to conserve power, so he planted gelignite charges in the holes and touched off the explosive at a distance with a battery. The circular section went up in smoke and larger fragments of rock. They rose higher than they would have on the home planet, the smoke disappeared more quickly, and the dust settled more swiftly.

'If there's an automatic sealing system, and it's still working, then the end of that tunnel will be shut off,' Orme said. 'And we'll have to open a door. But that will mean that the next section will seal. We don't have the materials to go through many doors.'

The tunnel, if it continued in a straight line, would go into the canyon wall. By now the shadow of the western wall was over them, and it was getting colder. They were comfortable in their suits, bulky though they were and with much equipment strapped onto them. Inside each was a flask of water which they could suck up through a tube by bending their heads inside the helmet down and to one side. They still had half a flask left, and they could urinate into a bladder attached to the front of one leg.