A single red light, winking slowly, as if it was the only thing that got the joke. Then, another, and another, five more, a crimson constellation in the darkness.

'Carter. You need to wake up.'

A man groaned, rolled over.

'No, I don't, Hatty. It's still the middle of the night. Check your clock batteries.'

'Carter, you need to wake up. We're being attacked.'

'We're all alone out here. You're imagining things.'

'I don't imagine things,' the AI replied, testily.

They had been lovers, once, had Hatty and Carter. It hadn't worked out; transhumanism had left him literally cold, and she had hated the confinement of a fleshy meat body. So, they had separated, fairly amicably. She had moved on, and was seeing a bulk freighter. He hadn't.

He flung the duvet off, sat up, yawned. The room lights came on.

'Where are the bad guys, then, Hatty? Have we put shields up?'

'Carter, take this seriously. Look.'

A display danced to life in front of him. He squinted, trying to make sense of it. With a click and a hiss, coffee started pouring into a cup to his left. The hot liquid steamed in the frigid air.

'Thanks Hatty. What's this map? I don't recognize it.'

'It's not a physical map. It's our network topology. It shows the ingress and egress points.'

'Oh, OK.'

They had been on this dark, silent world for nearly three months. The planet was a rogue, ejected long ago from its solar system by some cosmic accident, and was now gliding through deep space without a sun to warm it. Tragically, it had been inhabited, by a civilization perhaps half a millennium from achieving space flight. So now, buried under the snow that had once been atmosphere, next to the frozen seas and watched by the pitiless stars, Carter and Hatty were running an archaeological survey of this world-sized tomb. Alone.

'It's not your boyfriend, is it? Come to poke your ports?'

'For heaven's sake, Carter! First, they're not my boyfriend; second, they are weeks away...'

'OK, never mind then. What is it?'

'You remember the strange background radio signals we were getting?'

'Yeah, we thought they were something geological, right?' As he spoke, he picked up his coffee and walked into the main control room, wrapping himself in a thick dressing gown against the cold.

'Right. The signals have been changing, shifting frequencies. Earlier this evening, they matched our drone communication channels perfectly.'

'That's weird. Solar storm?'

Lights blinked around him, impatiently.

'Carter, drink your coffee and wake up. It can't be a solar storm because there's no sun.'

'Sorry, I forgot on account of how it's the middle of the damn night. Look. There's some funny noise out there. Maybe it's reflected space stuff. Can I go back to bed?'

'Half an hour ago the noise started mimicking drone communications.'

'Huh.'

Carter put the mug down and started paying attention to the network diagram in front of him.

'That's not actually an attack, though, is it?', he asked.

'It's classified as one. A low level one, of course, but definitely a sign of hostile action.'

'But there's no one out there, Hatty. Best guess is that this planet was rendered uninhabitable two hundred thousand years ago. No civilization with their tech level could survive that.'

'And yet, here we are. With someone performing a passable imitation of a port scan on us.'

'It's like that Clarke quote: either there's something out there or there isn't, and either way we're screwed.'

'That's not actually the quote, but it summarizes our situation pretty well. Carter, I am going to shut down the drones. I am worried that one of them might get compromised.'

He frowned.

'This thing is capable of copying our comms?', he asked.

'To a degree. It doesn't seem to understand the contents, so it's just sending random data that it's seen before.'

'And, if you shut down the drones, you will send the same request out to all of them? Like, generate the same message loads, just with different destination headers?'

Hatty paused. 'I hadn't thought of that. You're saying a repeated, simple message might be something it can copy more easily.'

'Right. And then we may never be able to bring them back up.'

'Damn.'

'Hatty! Outsmarted by a fleshy brain.' Carter took a celebratory swig of his coffee, and then swore. 'Ow, that's hot!'

She laughed. 'Maybe not so smart, then... Blow on it next time. Instead of shutting them down, I'm going to start sending them here. There is an upgrade I can perform which will make them much harder to hack, but it requires physical access. If our attacker copies the "return home" instruction, it's just making my job easier. It won't work after the upgrade.'

'What am I going to do, then?'

'You, my ape friend, are going to figure out where this signal is coming from.'

                                                                                        * * *

Carter sat back on the chair and stretched. The automatic morning cycle had kicked in, and the hab was slowly coming to life, lights fading from orange to white. In one of the domes, their bees were waking up and getting on with their day, which Carter found vaguely comforting.

'It's coming from the planet itself,' he said, mostly to himself.

Hatty had automated the drone upgrade process, and was giving him her full attention again.

'That's why we thought it was geological,' she reminded him. 'The planet's core is still hot. It has a magnetosphere and volcanoes and all the rest. That doesn't really tell us where it's coming from, though, does it? It's not a small planet.'

Carter shook his head. 'No, sorry; it's coming from the whole planet. In fact, it's coming from a set of points across the surface near or in cities. Places that were inhabited. Places where you would find people.'

'But the drones have dug all over. Everything is ruins and corpses and snow.'

'Yes. It's almost like the planet is talking to us.'

She paused. 'Are you implying that this is space ghosts?'

'I... it sounds less dramatic when you call it that, but... yes. Maybe.'

'Here's another quote for you: "when you have eliminated..."'

'OK, OK. So we haven't eliminated everything else, point taken. What do you think it is, then? Here's what I've found so far.'

While she digested the data, he stood, walked to the bathroom, and brushed his teeth. When he got back, she was quiet, thoughtful. He stared at his terminal as waited for her to be ready to talk.

'What would you do if you knew you were going to die?', she asked, quietly.

'Party. Like, incredibly hard.'

'No, after that. Especially if you knew you were going to die alone, far away from everyone else.'

He thought a bit. 'Write a note or something, I guess. Depends on how long I had. We've found some of those, though. What are you getting at?'

'Next; we've found geothermal wells, right?'

'Yes, yes we have. Particularly round the cities.'

'These people didn't have the tech to make true AI, but their compute power was pretty good. And here's the thing: we've forgotten a lot of tricks there. The same way you don't know how to make a steam engine or a petrol engine work to their fullest, because we have fusion. Those computer techniques have been lost, now we have intelligences like me. Well, maybe, just forgotten.'

'Wait a minute. You're saying...'

'This is a massive, primitive, geothermal-powered computer. It's been designed with the express purpose of communicating with whoever comes after the end, using the only power source that would last: the planet core itself. We woke it up, and it's using simple, heuristic-based learning to try and tell us it's here. And you know what I think it's purpose is? It's a gravestone to an entire species.'

Carter rubbed his eyes and sighed.

'Their last will and testament. The note to the people who came after.'

'That's right.'

'I read the actual Clarke quote. In the end, it was the same for them. It didn't matter if they were alone or not: both possibilities were terrifying. Because they knew they were going to die. But they were determined to leave their legacy. In case they weren't alone.'

They fell silent, the human and the machine both lost in their thoughts.

'Carter,' said Hatty. 'I'm going to try and start talking to it. See if I can communicate. Can I ask you a favor?'

'Sure, Hatty. Anything.'

'Be with me while I do.'

'Of course.'