Thermos FTW
Arista sat with the problem for the better part of a day, sketch after sketch discarded, working through everything she already knew about materials that could survive sustained, direct heat.
"...Canal stone's the obvious first answer," she said, half to herself, half to Bram. "Same stuff lining Buzzy's boiler wall. Naturally resistant to sustained heat, no exotic processing needed, proven at a civic scale for generations."
"But?" Bram asked, catching the hesitation in her voice.
"But it weakens under sustained water contact. That's exactly why the boiler's water-side wall needed a thinner stone layer backed by metal in the first place, canal stone alone couldn't handle prolonged exposure to both heat and water at once." She tapped the sketch, already crossing the idea out. "This tube's going to sit submerged, permanently, in an actively boiling ring. That's the worst possible combination for canal stone specifically. It'd degrade within weeks, maybe less."
She sat with the rejection a moment, then reached for a fresh page entirely.
"...Lead, then," she said slowly. "I don't use it often, but it fits everything this actually needs. This isn't direct lava contact, it's boiling seawater, sustained, permanent exposure. Lead handles that temperature without issue, doesn't rust the way iron or steel eventually would, stays watertight indefinitely, and it's workable enough to actually shape into something this size."
"Lead handles the actual barrier, then," Bram said, working through the logic. "Keeps the boiling water out entirely. What handles the heat that gets through the lead itself, though? Metal still conducts."
Arista considered that properly, tapping her pen against the desk. "...That's the actual open question. I don't have a good answer yet."
She sat with it a while longer, running through everything else she knew, and landed, almost sideways, on cloud material. It conducted electricity remarkably well, that much was already well established. Electricity generated heat, given enough resistance and sustained current. If it could handle that much without degrading, testing it directly against sustained heat itself seemed worth the actual effort, rather than just assuming.
She held a fresh sample against a heated iron rod, keeping the contact sustained for several minutes and watching closely.
The far side stayed cool at first. Then, slowly, a faint warmth crept through. By the third minute, a dull, spreading orange glow had started at the contact point, the material holding its shape perfectly, no melting, no structural failure at all, but unmistakably heating through regardless, the same way a bar of ordinary metal warmed gradually in a forge given enough sustained exposure.
"...Huh," she said, watching the glow spread further than she'd expected. "It doesn't conduct heat quickly. That part's true. But given enough time in sustained contact, it still absorbs it, slowly, and eventually gets hot enough to matter anyway." She set the sample down, letting it cool, genuinely surprised by the result. "That's not the same as actually blocking heat. That's just delaying it. Fine for a quick test. Not remotely fine for something sitting in direct contact with boiling water permanently."
"...Not cloud material, then," Bram said, watching the glow slowly fade.
"Not cloud material," Arista confirmed. "I almost reached for it purely out of habit, honestly. Good thing I actually tested it properly first instead of just assuming."
She logged out that evening, laptop open at her desk, chasing the actual problem properly this time. How to keep heat out.How to insulate a tube. A brief, entirely unrelated detour into a video of a cat relentlessly bullying an unbothered golden retriever, which she watched twice before catching herself and getting back to it. Then, finally, the term she'd actually been circling without realizing it: thermos flask.
She lost the better part of an hour to it, double-walled construction, a sealed, evacuated gap between an inner and outer layer, the whole principle resting on the simple fact that an evacuated gap removed almost everything capable of carrying heat directly from one wall to the other.
She logged back in the following morning with the answer already fully formed.
"...We're building a thermos," she announced, before Bram had even finished settling into his seat.
Bram stared at her. "...A what?"
"A six-meter thermos. For mermaids."
"...Explain," he said, already leaning in.
"Two lead tubes instead of one. An outer shell, and a second, sturdier inner tube, sized slightly smaller, sitting a short distance inside it. Seal the gap between them completely, airtight, both ends. Then pull every last bit of air back out of that sealed space until it's a genuine vacuum. Nothing left in the gap at all, no air, no gas, nothing solid touching both walls at once to actually carry heat across."
Bram studied the concept, working through it properly. "...An empty gap between two metal walls. No exotic material required anywhere in the design." He paused. "Both tubes lead, though? Seems excessive for the inner one, if it's not actually touching the boiling water at all."
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"It's not touching the ring's water," Arista agreed, "but the passage still has to stay filled with ordinary seawater on the inside, for the mermaids to actually swim through. Whatever forms that inner wall sits permanently against water regardless, just a different, considerably calmer kind. No reason to use two different materials when lead already handles both conditions without issue."
Claire, reviewing the sketch over Arista's shoulder, nodded slowly. "A proper vacuum cuts out conduction through the gap and eliminates convection entirely. What's left is mostly radiative transfer, and at these temperatures that's manageable."
"Then that's our answer," Arista said, already sketching the revised cross-section. "Outer lead shell, heavy enough to fully block the boiling water outside. Sealed vacuum gap behind it. Inner lead tube, sturdy enough to actually hold its own shape and take the structural load, containing the ordinary seawater the passage itself needs to stay filled with."
"And the interior?" Claire asked. "Sealed off from the ring's own water entirely, I assume, given what's actually boiling on the other side of that outer wall."
"Has to be," Arista confirmed. "The inner tube stays open at both ends. Normal seawater can flow straight through it from one side of the ring to the other. The vacuum gap is sealed; the passage itself isn't."
Bram studied the cross-section sketch. "So we're not keeping the water in. We're keeping the heat out."
Arista grinned. “Exactly.”
She turned back to the sketch, extending the cross-section into a rough side view of the full six-meter tube. Four broad platforms appeared above it, evenly spaced along its length, each connected to the tube by a short support cable. Beneath the tube, she drew four more cables descending toward the seabed, terminating in the same thick adamantium screw anchors they'd already used successfully before.
“We can't put the tube directly on the seabed,” she said as she worked. “Lead handles boiling water just fine. Active volcanic terrain is a very different problem. If we put this thing directly on whatever's heating the ring, there's a decent chance our six-meter mermaid thermos becomes a six-meter puddle.”
Bram leaned over the sketch, following the four cables down to the screws she'd drawn at the bottom.
“Straw, I don't think I can survive at that depth.”
Arista glanced up. “You did it before.”
“At a hundred and fifty meters,” Bram said. “We sailed the boat way past three hundred.”
Her pencil stopped.
She looked down at the four adamantium screws.
“...Right.”
“And even if I could handle the pressure for a little while longer, I'd still have to install four of those things while occasionally remembering that I need to breathe.”
Arista stared at the sketch for another second, then drew a firm line through the first screw.
“Okay. Nobody goes to the bottom.”
She crossed out the remaining three as well, replacing each with a much broader, heavier shape.
“Anchors instead. Nothing that needs to be driven into the seabed or installed once it's down there. We make them heavy enough to keep the tube exactly where we put it, lower them from above, and let gravity do the rest.”
Bram studied the considerably larger blocks she'd just drawn. “Those are going to be some very heavy anchors.”
“That's generally what I look for in an anchor.”
Claire had been studying the rest of the design in silence, eyes moving between the double lead walls and the four platforms now supporting them. After another few seconds, her expression tightened.
“Actually, that's not the part I'd worry about.”
Arista looked over. “What is?”
Claire tapped the tube itself. “This. Those platforms made sense when you were dealing with a single lead wall. You've doubled that now. Two full lead tubes, plus whatever internal supports you need to maintain the vacuum gap. This thing is going to be heavy.”
Arista looked back down at the four platforms.
“...Very heavy,” Claire added.
“Yeah, I got that part.”
She tapped her pencil against the page for a moment, then began thickening each of the platforms, drawing additional enclosed volume beneath their original flat profiles.
“Then the platforms need to do more than float themselves. Sealed air chambers underneath each one, sized against the finished weight of the tube. Enough displacement to keep the whole structure suspended at the depth we actually want.”
Bram frowned at the drawing. “Wouldn't that just make the entire thing try to float to the surface?”
“If I overdo it, yes.” Arista kept sketching. “So I don't overdo it. Enough positive buoyancy to keep the tube safely off the seabed and support its weight. The four anchor cables counter the remaining upward force and keep it fixed at exactly the depth we need.”
Claire's expression eased as the finished arrangement took shape on the page: four buoyant platforms keeping the tube suspended beneath them, four long cables disappearing toward heavy anchors far below, the passage itself suspended safely between the boiling surface of the ring and the volcanic terrain beneath it.
“That works,” she said. “Nothing has to be installed at depth, nothing touches the bottom except the anchors, and the tube stays exactly where we need it.”
Arista leaned back slightly, studying the increasingly crowded blueprint.
“Six-meter thermos for mermaids,” she said. “Now with flotation devices.”
Bram nodded solemnly. “Safety first.”
Arista snorted and looked back down at the blueprint, taking in what had started as a relatively simple problem and somehow grown into considerably more. The single lead tube she'd first imagined had become two, separated by a vacuum; four platforms would hold the passage safely above the volcanic seabed, their newly added buoyancy chambers carrying the enormous weight of the lead, while four long cables disappeared hundreds of meters below toward anchors that could be lowered into place without anyone needing to follow them down. It was more complicated than anything she'd pictured when she'd first sat down with the problem, but every piece was there for a reason, and for the first time since she'd started sketching, she couldn't immediately see another problem waiting to be solved.
Bram studied the finished design alongside her for a while, his earlier amusement gradually giving way to something quieter. “...This is genuinely good, Straw.”
“I know. I'm amazing.”
“That's not what I meant.” He pointed toward the open passage running through the center of the drawing. “If there are still mermaids down there, this isn't just another thing we've built. That's a way out. Maybe the first one they've had in who knows how long.”
Arista's grin softened as she followed his finger across the page, through the narrow corridor they'd spent all this time figuring out how to keep cool, suspended and exactly where it belonged. Until that moment it had been a materials problem, then a heat problem, then a weight problem, then somehow an anchoring problem too. She'd been so busy solving each one as it appeared that it was surprisingly easy to forget what the finished thing actually represented.
A door.
She sat with that for a moment, then finally rolled up the blueprint and tucked it beneath her arm as she stood.
“...Let's go build it, then,” she said, the grin returning as she headed for the door. “Let's give them their door back.”