Liu gapes at me. Several long seconds elapse before she says, "Did you just propose that we blow something up?"

"I did." I can't stop the tug of a smile. "Well? What do you think?"

Still stunned, she turns her attention back to the snow hill. "Well, the flares aren't electronic... I don't know how it would react. We've never blown something up here before."

"There's a first time for everything." That's a joke. "But that aside, how different would it be from the snow cannons or the defroster?"

"It doesn't like the defroster."

"I wonder about that."

She frowns at me. "Why?"

"I've been thinking about all the times Mahaha's reacted to us around the Pod, and what they all have in common." The trip here gave me lots of time to overthink things, so I figured I might as well put it to good use. "Name all the things it's done."

"It's attacked Dea..."

"Besides that."

She ticks them off, still frowning. "It's frozen a lot of instruments... it puts ice on the Pod... puts ice on the instrument panel... covers us in butterflies..." She gives me a sidelong glance. "What do those have in common?"

"Think about how we respond to them." I want to see if she comes to the same conclusion I have, based on the evidence.

"Well, Tobias has to go out and thaw the instruments, and you go melt and chip off the ice. We didn't really do anything about the butterflies."

"We did. The Pod's top camera was totally covered when we raised it to see if we'd been buried. The butterflies cleared when Dea turned on the heating mechanism."

I see the moment she lands on the answer. Her eyes widen. "Melting the ice."

"All of those make us go out and melt whatever it puts on our equipment. And then every time we melt it, it puts more back in the same place."

"Like it wants us to do it again." Liu is staring at me. "Do you think—"

"I don't know," I say. "But I think this moon of ours might be fond of water."

"But then why did it back off when you and Tobias hit it with the defroster? When you were rescuing Dea..."

"What if it wasn't the defroster?"

On the day we woke up to find the Pod covered in butterflies, Krüger told me Mahaha acted differently towards him. If it likes the water that comes from us melting things, that would make sense: he's been outside more than the rest of us combined, checking our instruments twice a day and thawing them again and again. I saw snow fingers reaching up the trickles of meltwater one such trip sent running down the Pod wall. Reaching like they wanted to catch the drips.

I've been out less often, and until recently, I've had no association with water. It wasn't until Kwon invented the defroster and we began using it everywhere that I found myself with a heat cannon in my hands, accompanying Krüger around the Pod to check for storm damage. We melted and chipped small patches of ice off the wall. The next night, we found that same wall covered in out-of-place ice. Krüger and I went out to melt it off again. It was on our way back that Mahaha first scoped me out with a butterfly.

The parallel doesn't stop there, either. There's no other reason I can think of for Mahaha to discriminate between our technologies, besides the obvious example of a person it likes sitting on a vehicle it doesn't. Our probes were an easy target. But the Pod, with Krüger thawing his instruments and both of us melting off ice, has been a ready source of water.

Water I'd be willing to bet the last mission to come here never had a chance to produce.

And water that, when I locked everyone inside, disappeared. Only then did Mahaha attack the Pod.

"I think Tobias is right that it favors him," I say. "And I think it feels the same about me now, too. I think that's why it backed off when either of us threatened it. And why we've always driven around safely, while Dea alone got attacked."

Now Liu's frowning at the snow hill. "But why does it like water?"

It's a good question, and I don't have an answer to it.

Liu does, though. The next moment, she claps both hands over her mask. "The Neuron Theory. There's a living ecosystem somewhere here, if Mahaha is conscious. There has to be."

Of course.

Living systems can't feed straight off the ice. Even on the icy planets and moons where life has been found, it's all had some way of converting frozen water into liquid for its own use. If Mahaha has life, it's far below the surface. It's also extensive enough to generate all the methane the sensors in my goggles are picking up, streaming from every crack and pore of the snow hill ahead. If Mahaha likes Krüger because it associates him with water, and it needs that water to feed its biological brain, I'm willing to bet it's taken him to the heart of all this: the very neural network that makes Mahaha alive.

I'm really glad I brought extra rope.

"I don't think it will hate us if we blow something up, then," says Liu, another conclusion that suddenly makes perfect sense. "The fire will melt at least some of the snow. Kind of like a peace offering."

"Let's do it, then."

We spend the next few minutes plotting, complete with diagrams and a meteorologist's evaluation of the physics of gas dispersion in a turbulent atmosphere. At one point, I brace myself, fight back every habit of the last nine years, and walk close enough to the hill to get a reading on all the gases leaking from it. Liu is delighted by the findings.

She summarizes her conclusions for me in plain language. "I think there's a slight risk of the fire travelling down the hole, but the rest of the gas coming out is mostly carbon dioxide. It'll suffocate any fire that goes too deep, so whatever's at the bottom is probably safe. I don't think it'll burn on its own up here, either. It'll use up the oxygen stored in the flare, then die out."

I'll take her word for it. "Do you think there's enough to get us an explosion?"

She rocks back on her crampons, crouched beside a calculation in the snow like a snow-suited marshmallow. She scribbles a few more numbers. "I think we should concentrate the methane."

"Shouldn't be too hard."

And with that, the plan is complete. I hack into the side of the snow hill, widening the biggest leak. Liu runs around patching every other hole she can find.

"I think that's good enough," she says when the gas jet out the escape vent I'm making is strong enough to blow snow. She circles the pile one last time. The reading on my goggles has passed the threshold she proposed as a required minimum for explosion.

"Then we're good to go."

We retreat as far as I know I can lob an emergency flare, and shovel ourselves a shield-wall of snow and ice. I check the flare over. All exoplanetary flares in the UIS are built with their own oxygen supply, so they'll burn in any atmosphere. This one's intact and, by the weight, has maintained a full tank. They leak sometimes. I yank out its ignition pin, count to three, and square the long-distance throw directly into the crater I excavated around the methane leak.

There's a twinkle, then a brilliant flash. Snow goes flying as a fireball taller than the Pod punches a hole in the side of the snow hill. It's gone just as fast. I make Liu wait until we can be sure the smoking relic of the flare has no more oxygen left to give, then cautiously sneak back to it. The hole we blew is big enough to crouch in. Its other end opens into darkness.

I send Liu back to where we dropped our backpacks. She returns with a headlamp. I fasten it to the mount on my helmet and turn it on.

Underneath the snow hill is a hole some three feet across, as shiny-sided as if it was made of glass. I tell Liu to keep watch, then drop onto my stomach and inch forwards until I can peer over the edge. I'm hit in the face by a blast of fine snow. From below? I shield my face, waiting for the flurry to stop, but it never does. I slowly lower my hand again.

Staring back at me is a tunnel sunk straight into the ice, curving gently to the side until it splits, splits again, and is lost from sight some twenty meters down.

Fuck.

The smooth ice bears no trace of Krüger's fall. Not a scuff, not a scrape, not a dislodged lump of snow. "Tobias?" I shout down.

There's no reply.

"Tobias!"

"What is it?" says Liu from outside, worry bordering on dread in her voice.

I fumble for the whistle around my neck, slip it under my mask, and blow a shrill blast down the hole. Nothing responds, so I try again. Then again.

Silence.

The whistle nearly falls from my hand. My mind is pinwheeling, but my body is frozen. Paralyzed. Do I let Liu see this? She's smart; she'll know the impossible problem we now face from the moment she looks down the hole. If this tunnel keeps branching, there's no way to know which route to follow. If Krüger is too far down to hear even the whistle, or at least to reply, we're never going to find him.

He's gone.

I backtrack slowly and stand up. Liu's face pales as she sees the look on mine.

"Can I see?" she says.

This isn't an option I imagined when I played out possible scenarios. I want to shield Liu from the potential reality we now face. I don't want her to think it's her fault that Krüger fell. I've barely lived with that kind of guilt. She would break completely.

"Can I see what Mahaha did?" she says, and the tone in her voice makes me look up. Her expression is calm.

Or maybe she has the right mindset to avoid the pit I fell into altogether.

She and Krüger were close. She deserves to know.

I let her approach the hole on her stomach while I keep watch. She lies still for a long time, just staring over the rim. She's just shifted to move back again when something on her starts beeping.

"What's that?" I say, a shot of alarm coursing through me.

She retreats from the hole. I wilt in relief; it's not a warning. It's the receiver. Liu unclips it from her jacket front as the sound stops again. She gives the screen a nonplussed look. Then she shows it to me.

It's the Isoptera's signal.

The Isoptera is dead. Its remains are locked in Mahaha's ice or back at the Pod, just scraps of wire and metal.

But it's not the body of a probe that sends this signal. It's the transmitter pod.

And then it clicks.

"The transmitter pod," I say. The world tips, and I brace my feet to keep myself from reeling. Relief stronger than a waterfall cools the panicked rush that only moments before was about to consume my world.

"Did he have it?" Liu's voice has a tremble in it that her mask doesn't cover.

"In his jacket pocket."

We can find him with that. The smooth walls of the tunnels will reflect the signal, guiding us down whatever tunnel we need to follow to trace Krüger's path. I stumble as Liu crashes into me. She wraps me in a hug, not managing to stifle a whimper near my shoulder. I hug her back. Tightly.

"We'll find him," I say. It's a reassurance to both of us. "Let's get the climbing gear. We're going down that hole."