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In this question, I asked if it would be possible to make a city like Venice, but with lava instead of water. The answers I got showed that the city would be very impractical, due to toxic gases and other things.

But, is there anything that could replace the lava, so there are still hot streams of liquid below the city?

I have some guesses, such as boiling springs or molten metal, but I don't know what would be the safest. It does not need to be safe to fall into (I want it to be quite the oppisite actually), it just needs to be safe to live above.

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  • $\begingroup$ Can we handwave how we are feeding this hot stream? $\endgroup$ – Bellerophon Nov 30 '16 at 22:18
  • $\begingroup$ A Zirconium Oxide coated Titanium boat would float quite happily in molten lava, it'd just be terribly expensive, but the insides could all be steel (ZnO2 an Ti both have rubbish thermal conductivities so they work well as thermal insulators) $\endgroup$ – Samwise Nov 30 '16 at 23:11
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    $\begingroup$ @AndreiROM To all duplicate voters, this is an acknowledged follow-on question to the one you are declaring it a duplicate of. Please stop. $\endgroup$ – kingledion Nov 30 '16 at 23:34
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Your safest option is probably boilling hot water. If the water is pure, unpolluted water then heating it will produce no toxic fumes/gasses and no dangers to life (unless you fall in then you could get severe burns). Furthermore there are a lot of materials that can cope with 100 Celsius (212 Fahrenheit) meaning building would be fairly easy as you could just use some kind of stone. Water isn't that exciting though and boiling Venice would just be Venice with steam.

Your next option is molten metal. The easiest metal to get molten is mercury but we want hot liquid so we will go with aluminium, Earth's commonest metals. Aluminium has a melting point of 660 Celsius (1221 Fahrenheit) which is hot but still below the melting point of some rocks so building won't be impossible. Pure molten aluminium is safe to breathe near as well. Other metals can be used if you like but I would be careful using heavy, transition metals as some produce toxic vapours and others have such a high melting point you will struggle to build near them.

I think you should go with liquid rock though as this is kinda similar to lava and also is less mainstream than liquid metal. Lets go with granite as it is igneous so it is formed when molten. Granite melts at about 1260 Celsius (3000 Fahrenheit) when dry but this is lower when wet. If you mix some water into your granite stream you can reduce the temperature required to around 600 Celsius so that is good if energy is an issue. I'm not certain that their is much that is still solid at 1260 Celsius so you may need to mix in a little water in order to be able to build your city. One small problem with granite is it can contain radioactive elements such as thorium and radon but it is unlikely that the radioactive radon gas produced from the granite will build up outdoors so the granite should be safe.

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  • $\begingroup$ Radon gas produced could however seep into buildings and build up within them, just as it does in some areas like Cornwall in th UK. $\endgroup$ – Sarriesfan Nov 30 '16 at 23:44
  • $\begingroup$ I hope you don't drop anything made of iron into your molten aluminum, because thermite is quite explosive. Nothing like getting getting molten aluminum globbed all over you after dropping your hammer into the canal by accident. I'd use something more inert, like Zinc or Tin. $\endgroup$ – kingledion Dec 1 '16 at 14:20
  • $\begingroup$ Problem of aluminium is that you won't find a lot of it in nature in pure state, it needs to be electrolyzed, which is not kinda natural process. So using aluminium is not really realistic. $\endgroup$ – Antoine Hejlík Dec 1 '16 at 14:57
  • $\begingroup$ @Sarriesfan It could but you can build the buildings to negate that. $\endgroup$ – Bellerophon Dec 1 '16 at 16:23
  • $\begingroup$ @AntoineHejlík I don't think we find much super hot metal, pure or not. $\endgroup$ – Bellerophon Dec 1 '16 at 16:24
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Could it just be hot water? (Possibly hot enough to be dangerous? and/or poisonous?) You might put the city in a mostly-dormant caldera -- the key here is that mostly. A city either on the shore, or on an island, surrounded by the volcanically-heated caldera lake. (Crater Lake in Oregon has such an island, but is either extinct or very dormant. Check it out. ;-)

I have a vague memory tickling me about this. I seem to recall some sort of disaster associated with a caldera in south America, but can't recall the name.

Now as for why they'd build a city there, you've still got some work to do.


Long term, I think most metal vapors are hazardous (see Smelter's Flu.)
We've got some options cooler than Aluminum (and thus might have lower vapor pressures?) How about Bismuth? melting point 271.5 °C, ​520.7 °F?
Less if we throw in the right alloying metals, but most of those are toxic. Bismuth is relatively more safe, at least chemically. (BeptoBismol, for instance.)

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  • $\begingroup$ Only some metals cause Smelter's Flu. I think Zinc and galvanised metals are worst. Aluminium fumes and dust have been linked to drops in response times but I don't think it is that major. $\endgroup$ – Bellerophon Nov 30 '16 at 22:55
  • $\begingroup$ If you've ever been to Lassen NP, they have boardwalks going among some of the thermal areas. Same in Yellowstone, IIRC. But neither is a place that I'd want to spend much time in. $\endgroup$ – jamesqf Dec 1 '16 at 4:37
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I think lava is fine. If this is scifi-ish, maybe everyone wears gas masks, or genetically engineer themselves to be immune. If it's fantasy, maybe they wear amulets that let them breathe the fumes. Or maybe it's a race of Dwarves that have adapted to the heat and the toxic gases!

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  • $\begingroup$ This doesn't really answer the question $\endgroup$ – kingledion Dec 1 '16 at 14:17

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