Liu is so nervous, she keeps fumbling over simple knots, so I'm the one who sets up most of our rappelling gear. The ice in the tunnel is thick and hard. I don't foresee any problems anchoring ourselves directly to it, but I worry what it'll be like further down. These tunnels clearly don't stay cold all the way to the bottom, and we have no idea how far we'll have to descend. The Isoptera's signal is about the weakest I've ever seen still making it to a receiver. If the probe had been any lower-quality, I'm sure its transmitter pod would not have managed this range at all: we're so far away, the estimated distance between us and Krüger reads as nothing but an error.

Liu has barely said a word since we first found the signal. I do see her checking readings on her goggles, though, and at one point she pulls out a tiny pocket notebook to jot something down. I suspect we've discovered where Mahaha's methane pockets and the missing water in its system both come from. The tunnel, as I work at the rim of it, continues to spit up a constant stream of snow. The crystals are tiny, freshly formed. When open to the sky, this thing must serve as a chimney, spitting vapor back into the clouds.

I wonder how many of these there are on Mahaha.

Finally, I rope myself into climbing gear, pull my backpack on for extra weight, and swing myself over the tunnel rim to test our first anchor in the ice. My crampons bite the tunnel's smooth sides, flaking off thin layers like chips of mica. This ice must all be deposition from the moisture blowing up from below, pulled from the air and built up millimeter by millimeter over decades, if not millennia. It almost feels like a shame to send an ice screw into such a pristine formation, knowing the scar will stay there for half of forever.

The ropes hold. I descend carefully to the first tunnel split, noting everything around me in the claustrophobic space. I hope these passages don't get narrower as we go along. The receiver now clipped to my jacket—and strung on a cord around my neck for good measure—beeps softly, sometimes intermittent, but by-and-large consistent so long as I don't block its line of transmission. At the tunnel split, I hold it out over one of the two branches. It goes silent. I move it to the other branch, and the beeping returns.

"It works!" I call up the tunnel. There's a sound of relief from Liu above. "I'll take the rope down the right path, and call you when you can start coming down. Are you okay to start rappelling on your own?"

"I think so..." Her voice wavers.

"Don't worry," I say. "I'll walk you through it."

Like I so long ago promised to do when I offered her a place on this team. That guilt weighs me down until I close my eyes, acknowledge it, too, and stow it away. I'm not the legend she believed I was when that invitation was extended. I've fucked up, broken promises, and gone back on my word. I've denied her—and Krüger—the guided freedom this mission was supposed to entail, and while I started to give back the training element in the same sessions that taught Liu rappelling, I've got a whole lot more to make up for.

That's going to take another apology.

But for now, we need to focus.

"Stay in touch over your headset," I say. "We'll test how far apart we can be before they stop communicating."

I hop off the wall and glide down the rope into the tunnel with the signal, feeding out more rope as I go. It's another six or seven meters before I find the next split. I'm deep enough now that the already poor sunlight from above has been reduced to a glimmer off the tunnel walls. I switch my headlamp on and dial it down to the lowest setting. Its beam flashes up and down the ice. The tunnel walls are transparent for a good twenty centimeters in, their internal layers refracting the light and sending rainbows scooting through them. It's like the whole place is made of diamond.

The split this time isn't vertical. The tunnel I'm in continues downwards, but the branch forks off to the side like a den in an animal burrow. I push myself out from the wall to peek into it. It continues horizontally out of sight. The receiver signal becomes intermittent when I hold it as far down the branch as my arm will reach. I think it's safe to say Krüger went down the shaft I'm currently hanging in.

The existence of a horizontal shaft, though, raises a new possibility. If there are more of these, and Krüger managed to catch himself on one, he might have stopped his fall: an encouraging prospect. He's certainly quick enough to have pulled off a maneuver like that. I speed up a little, searching for more sideways branches. The tunnel I'm in has tipped from vertical to roughly the angle of a high-speed waterslide when I find the first mark in the ice.

It's a perfectly straight, hair-thin scratch, running for about half a meter before vanishing. I turn my light down the tunnel. Not far off is a bigger scar: deeper and longer, like Krüger found something to ram into the ice to try to slow his slide. I scoot down to it. It runs for a good three or four meters before breaking off sharply. There's something embedded in the ice at its bottom end.

I chip out the dark fragment and turn it over in my palm. It's the tip of a black-bladed, ceramic buck knife.

I gaze up the tunnel again, trying to estimate how long it would take to fall this far. Krüger must have gotten over the initial shock and gotten his knife out in seconds, stuck it in the ice, then slipped again when the tip broke. The scar isn't deep enough to have stopped or even seriously slowed his slide, but at least he was alive, mentally present, and trying.

I should probably show Liu this. I can hear her breathing in my headset; we're still connected. "Lingmei?"

She startles. "I'm here."

"How clear's the transmission?"

"You're a bit fuzzy, but the volume is okay."

"Good. Listen, I've got a mark down here that I want to run by you. Are you good to come down?"

"I think so."

The extra training I threw in after their workouts pays off. I stand by to help walk Liu through the rappelling process, but she only needs one prompt before she's gliding more or less smoothly down to meet me. She loses her grip not far above my spot and takes the last few meters in an unintentional butt-slide. I lean back on the rope to stop her, smiling.

"Sorry," she says.

"Happens to everyone on their first time. Use your crampons to flip back."

She digs in her heels to test it, then rights herself awkwardly. "I keep forgetting I have claws on my boots. What did you find?"

I point her to the scar on the ice, then show her the broken knife-tip. She comes to the same conclusions I did.

"I saw a horizontal tunnel back there, too," she says. "He could be camped out in one of those, if he managed to get to one."

"That's what I'm hoping. I'm also hoping he's stayed put if he did."

"He's always told me that the first thing I should do if I get lost out here is find shelter, and that the second thing to do is stay there, so I think he would."

That makes me feel better. And worse again, but mostly better. If this were the Aventureros, Krüger would be in leadership training, and he's certainly ready for it. Or was. If we find him alive and make it out of here, I'm going to ask him how he feels about stepping into a role as my second-in-command in the field.

"Are we going to keep going, then?" says Liu, eyeing the beeping receiver.

"Yup. Same pattern, and keep an eye out for other signs."

In another few minutes, we've lost natural light entirely. The trip now takes on a caving feel, all silence and slippery surfaces, punctuated by tricky patches or the interruption of a headset exchange. Teaching Liu how to increase the efficiency of her motions doesn't take long. Nor does any other coaching; she remembers what I tell her. In the silence, I become acutely aware of the beep of the receiver. I turn its volume the rest of the way down, then turn it off. Even Liu stops moving when I call for Krüger at each tunnel intersection, but only the sound of my own heartbeat replies.

Liu and I fall into a rhythm. We meet up on flatter stretches to talk and pull down the rope from the last rappel. Then I drill a new anchor into and through the ice, test it for stability, and start on the next stretch. When we run down to our last three ice screws, I consult with Liu and take to anchoring through the ice only. We keep our ice axes handy in case the tunnel wall gives.

I check the clock on my goggles for the first time when I start to feel more than physically tired, and am shocked to find us late into the evening already. There's no reference point to tell us how far we've come—we lost the mini-probe's signal from above after the first hour and a half—but if I had to guess, I'd say we're at least a couple kilometers down by now. If the radar data the Isoptera once collected is to be believed, that's not even the halfway point to the bottom of Mahaha's ice.

I weigh the value of continuing versus stopping for a rest. Or at least proposing one. I have the capacity to run without sleep for a pair of days, but Liu has neither that training nor experience, and exhaustion down here risks critical mistakes. If we're as little as a quarter of the way through this round trip, we need to pace ourselves.

Liu and I rendezvous again. "So, it's now past the tenth beat," I say, and she does a double-take that confirms I'm not the only one who lost track of time. "We have two options: keep going, or stop for the night."

She frowns. "For the whole night?"

"Well, 'night.' For food and a nap. Normally I'd press on, but we could be in for a long one here."

She slumps back against the ice. "Food sounds good," she admits. "And I am getting kind of tired."

"Me, too. Let's find another horizontal branch, then."

Those aren't common in this stretch. We descend for another hour before rounding a corner to find a sight that makes all my senses sharpen. In front of us is a stretch of tunnel that's basically flat, struck down the middle by the deep gouge of a broken knife-tip. This only deepens at its sudden end, rather than tapering off in a streak. Krüger finally managed to stop his fall.

Off to the side of the flat section is a horizontal tunnel mouth packed with snow. Its front is dented with the print of a shoulder, and more knife scars cluster the floor and walls all around. I tell Liu to wait, then approach the spot slowly. The snow has no sheen on the front of it. This blockage is fresh.

"Come help me dig this out."

Between us, we excavate the horizontal tunnel. I break through first, to find an untouched trail of snow all the way down the shaft. Krüger must have sheltered in this tunnel, but Mahaha didn't seal him in. It pushed him out.

"I found more knife marks," says Liu when I return from scoping out the backside of the blockage. She points over the edge where the flat section of the tunnel ends.

I glare at the ceiling, tapping the wall with my axe handle. "You're a petty bastard, you know that? You really didn't want to make this easy for us."

"She must really like Tobias," says Liu glumly.

I was wondering how long it would take for someone to personify the moon. "She?"

"It feels weird to keep calling her an 'it.' And she feels kind of like a 'she' to me. I dunno."

Language is a funny thing: even the simple transition shifts my perspective and makes Mahaha feel more like a living entity. I shrug. "Works for me."

We sling down our packs in the tunnel, keeping them on leashes in case Mahaha decides to evict us with a snow wall, too. I'm glad I packed for an overnight stay. Liu and I eat a proper meal bite by bite under our masks, then weigh the risks of sleeping without someone on watch. I'm a light sleeper anyway, and she points out that we probably want to conserve headlamp batteries if we're to keep going deeper tomorrow. In the end, we pull our backpacks around as pillows and make a bed from the emergency blanket in our gear. It's not particularly comfortable, but by now I'm too tired to care.