“See if you can beat that,” he grinned and looked at Montreaux.

But Montreaux wasn’t listening. He was still looking down at the flat rock that the stone had struck; the dust had now settled and despite its distance, it was obvious that there was a striking difference in colour between the surface dust that had been disturbed and the rock beneath. It was jet black.

“Captain Marchenko, pass me the binoculars, please.”

He focussed the electronic device on the stone below. After several long minutes, he passed the binoculars back.

“Look at the flat rock you just hit,” he said, seriously.

Danny’s first instinct was to be sarcastic. He felt like saying that he was sorry he’d damaged a Martian rock, but that there were billions more identical ones where that came from. Instead, he took the binoculars and steadied the image on the flat rock.

“What do you think it is?” He didn’t move his eyes from the binoculars, but instead zoomed in further to examine the small patch of jet black stone that had been uncovered. They had spent days picking up rocks and digging test holes in the soil near their base, and had never seen anything like it.

Montreaux had already been thinking about his answer, and had remembered some basic facts from his early days at school. “Well, on Earth, a black colour in stone often comes from carbon.”

Marchenko pulled his eyes away from the binoculars and looked at the American.

“Carbon? From plants and animals?”

“Yes, I think, although I don’t know if it can be naturally occurring too.” He thought about this for a moment. “In any case, we’ll never know until we get down there to look at it. We’ll need a sample to take back to Dr Richardson.”

“Wow! That was one lucky throw!”

“It certainly was. Now, I propose that we climb down together, but first, we need fresh air, I only have half an hour’s worth left.”

They climbed down the cliff in parallel, connected by a thin strand of synthetic rope for safety.  It was by no means a dangerous climb, and on Earth could certainly have been attempted with barely a second’s thought. But on Mars, the combination of reduced gravity and airtight suits made for a nervous descent for both men.

As they came within twenty metres of the flat stone, Montreaux paused and turned his upper body round to face it. He tugged sharply on the safety line to get Danny’s attention and pointed down at their goal.

“It’s definitely not your standard Martian rock formation, is it?” he said, out of breath.

“There’s something out of place about it, definitely. Something –”

“Not natural,” Montreaux finished his sentence for him, “I agree.”

From where they now sat, the stone betrayed not only its huge dimensions, but also its unexpected shape and orientation. It was about six metres wide and rectangular, protruding at least ten metres from the cliff-face.

Although it was difficult to tell with any accuracy from where they were sitting, the stone looked to be completely flat amidst the chaos of debris deposited around them.

“It looks almost like a,” Danny searched for the word. “Like a jetty, where you moor a boat on a riverbank.”

The two astronauts looked at each other and then continued to descend in silence. Montreaux checked the small LCD display on his forearm to ensure that his suit camera was capturing the whole event.

The Russian reached the stone first. He stood up to his full height and looked at Montreaux, who had continued past the stone and was now investigating below.

“This is incredible,” he said. “The stone comes right out of the cliff, it’s about a metre and a half thick, below is filled up with soil and other rocks, but apart from that you can clearly see the shape of the stone.” His voice was filled with awe. “This stone has to be artificial. The edge is perfectly flat all around, and wait.” His gloves had a built in rubber wiper along the seam of the thumb, for his own visor in case of sandstorms. He used it to scrape the stone clean. “Yes, that’s incredible! There’s a thick coat of dirt and dust, but underneath it’s the same jet black stone, and it’s so smooth!”

Danny was using his gloves to expand the impact crater that his small projectile had created.

“Same up here; it’s perfectly smooth. And whatever it is, it’s –”

Montreaux waited for him to finish his sentence, but the words didn’t come.

“Yes?” he pressed.

“Oh, my God,” Danny said, followed by a short burst of Russian that Montreaux didn’t understand. “Yves, get up here and look at this!”

Despite his usual friendly carefree attitude, Marchenko never referred to Montreaux using his Christian name.  That was a break in protocol that only Dr Richardson enjoyed. Montreaux almost fell over himself reaching the top of the stone to join the Russian.

“What?” he said, urgently, glancing sideways at the stone.

He pointed to the patch of black surface he had uncovered. His hand was shaking visibly.

Captain Montreaux did not need it spelled out. What had stunned Danny had the same effect on him. His knees grew weak, and he sank down slowly until he could touch the stone. He ran his gloved fingers across it until they slotted neatly into a groove, about two centimetres deep and ten wide that ran in a perfectly straight line from under one edge of the cleaned area, near the wall of the crater out towards its centre. Danny’s gloved fingers had left small furrows where he had done his best to clear out most of the sand. After about sixty centimetres, the groove split in two in a perfect V. At the edges of the cleared area, he could make out the lines of two more possible grooves. He stood up and tilted his head as he examined the stone.  The grooves made the shape of a V, pointing towards the crater wall, with a line emerging from its apex and disappearing into the cliff.

The grooves had been perfectly, and unmistakably, carved into the stone. And both astronauts immediately interpreted them in the same way: whatever had made the lines had intended them to point towards something that was now buried under the rock and debris of the cliff-wall.

They sat in silence for several minutes before Montreaux regained his composure. He pressed a button on his communicator pad and hailed the MLP.

“Dr Richardson, please confirm that you are getting the data feed from Captain Marchenko’s and my helmet cams.”

There was a momentary pause.

“Hi there. What am I looking at?” Jane said in a confused voice.

“Please confirm that you are getting this data feed and that it is being stored correctly, Dr Richardson,” he said plainly.

 “That’s affirmative, Captain Montreaux, both feeds coming in, there’s a bit of atmospheric interference, a little worse than when we spoke earlier. But what the hell am I looking at?”

“We are out of your line of sight, I imagine that is causing the interference,” he explained. “Can you give a quick assessment of the material we are standing on?”

There was a long silence, after which the scientist spoke quite cautiously. “I would say from its colour that it could be an igneous rock, it resembles obsidian.”

“Igneous? Obsidian?” Montreaux asked.

She sighed. “Igneous rocks are basically cooled down magma that on Earth forms most of the crust,” she elaborated. “Basically, magma leaves the mantle, normally during movement of tectonic plates or during a volcanic eruption, for instance, and solidifies as it becomes part of the crust because it is further from the heated core of the planet. In theory, the closer the magma is to the surface when it hardens, the faster it will do so because the ambient temperature will be lower. If magma cools slowly, the solid crystals that form can easily be bigger than your fist. As the rate of cooling increases, however, so the size of those crystals decreases. If the magma is on the surface, what we would normally call lava, and the environmental conditions are just right, it can cool so fast that crystals hardly have the opportunity to form, in which case we get obsidian. In these cases, the crystals have to be observed under a microscope.”