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The United States (as well as many other countries) is covered in a vast network of tubes:

enter image description here

These tubes vary greatly in diameter. Some could hardly fit a baseball, while others are large enough to comfortably fit a prone human.

enter image description here enter image description here

In a future world of renewable energy, where this natural gas infrastructure is no longer needed for fuel transportation, how could these tubes be repurposed?

If your proposal requires pipelines to have a certain diameter, please take into consideration how prevalent that pipe size is when explaining the reach and impact it would have. I have used the United States as an example in this question, but if you are more familiar with the natural gas infrastructure of another country you should feel free to use that country as part of your answer instead. For bonus points*, include a consideration of contaminants that linger inside these pipes (likely not food-safe, for example), and how that might impact your proposal.

*Disclaimer: not actually worth extra stackexchange points.

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    $\begingroup$ Not enough for a full answer, but at least some of them could be repurposed to remove household waste, and also there is a theory i saw long ago, can't refind it now, where some researchers suggested the now big empty caverns previous filled with gas could be used to store excess household/industrial CO2, so the pipes could fairly easily be reversed to take the bad household gases back to the pumping stations and into these CO2 resevoirs. $\endgroup$
    – Blade Wraith
    Jun 7, 2018 at 13:51
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    $\begingroup$ @BladeWraith: Carbon capture and sequestration is a scam, in my opinion. The total energy expended in capturing and storing 1 MT of $CO_2$, would be generated by power plants and engines that produce several times that amount of $CO_2$. Unless there are major carbon-neutral energy sources available, CCS is... well, even a dead duck is usable...so not even that $\endgroup$
    – nzaman
    Jun 7, 2018 at 14:07
  • $\begingroup$ Pneumatic tubes? $\endgroup$
    – nzaman
    Jun 7, 2018 at 14:08
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    $\begingroup$ Reminder to close-voters: The problem cannot be fixed if the OP is not made aware of it. $\endgroup$
    – Frostfyre
    Jun 7, 2018 at 16:09
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    $\begingroup$ A bit too flip for an answer perhaps, but Belgium actually does has pipelines that deliver beer to pubs. Why not repurpose them to that end? $\endgroup$
    – ohwilleke
    Jun 8, 2018 at 2:20

14 Answers 14

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They can used for underground power and communication lines, as a replacement for overhead lines.

The reason why overhead lines are still prevalent in America is due to the initial cost of installing the lines underground and the sheer number of lines needed for the sparse rural areas covering most of the US. However, if the power companies had available pipes across the country to simply snake the power lines through then the installation costs should be significantly lower.

Having underground lines would lower the number of outages due to not being affected by strong wind, falling branches, or drunk drivers, so the power companies would likely support it. It would also improve the view of most areas due to no longer having interconnected monoliths cutting across the landscape, so environmental groups would likely back it as well.

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    $\begingroup$ This is excellent. It also makes intuitive sense that many (but of course not all) of the routes currently taken by natural gas pipelines might one day be useful for electrical transmission. Wind/solar/hydro isn't always produced in the same locations as natural gas, but sometimes they are, so there is definitely potential for network synergy. $\endgroup$
    – Pink Sweetener
    Jun 7, 2018 at 17:03
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    $\begingroup$ This is/was already done NY Times Article, so it isn't a crazy idea. $\endgroup$
    – hazzey
    Jun 7, 2018 at 20:56
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    $\begingroup$ Williams Oil and Gas out of Tulsa established Williams Communications on this basis. $\endgroup$
    – pojo-guy
    Jun 7, 2018 at 22:14
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    $\begingroup$ So what you're saying is: in this future, the internet is a series of tubes. $\endgroup$
    – Pharap
    Jun 8, 2018 at 1:33
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    $\begingroup$ @jpmc26 Fiber can be laid without any containment other than its own sheathing. Dropping it inside a steel pipe with 1/3rd inch wall thickness is so much overkill it's not even funny. From the fear of explosion perspective, fiber is one of the few ways it is safe to get a signal to devices that are in an explosive atmosphere environment. $\endgroup$
    – pojo-guy
    Jun 8, 2018 at 11:51
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It is actually a trick question, in a fully renewable energy economy natural gas pipelines will continue to transfer natural gas produced from renewable sources.

Natural gas normally comes from fossil fuel sources, but at its most basic it is methane which can be renewably generated from the decomposition of organic material (such as food wastes or other organic garbage).

There are a number of operating biogas producers using large scale digesters (often common for waste processing plants or dairies) which mostly use the gas on-site. In a post fossil fuel world it may make economic sense to have larger scale processing centers and use existing pipelines to distribute the gas for usage.

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    $\begingroup$ Yup, Fossil fuels continue to be widely used as they’re very effective for transporting energy, and it’s easy to build and maintain a device that’s uses them. But this isn’t because they’re fossil fuels, but because they’re hydrocarbons. There’s no reason a renewable fuel system wouldn’t use hydrocarbons. Our local buses use a renewably produced LPG equivalent. $\endgroup$
    – Dan W
    Jun 7, 2018 at 18:29
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    $\begingroup$ I can't tell if you're being a pedant about the fact that I was careless in equating "renewable energy" with "GHG neutral", or if you really believe that bio-fuels are a promising solution to climate climate change. The idea that bio-fuels can be a renewable, net-carbon-neutral step in the carbon cycle is not really taken very seriously by most economists and environmental scientists anymore: feedstocks just require too much land and energy, and (depending on the waste product) there either isn't enough waste, or it's cheaper to reduce waste rather than to turn it into fuel. $\endgroup$
    – Pink Sweetener
    Jun 8, 2018 at 4:10
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    $\begingroup$ @PinkSweetener If you're concerned about carbon, it's entirely possible that some other way of dealing with carbon emissions can be found. Maybe there's a way to absorb carbon out of the atmosphere, making the process net neutral, or maybe a way of capturing it before it was released is developed. There's no reason to assume that the only solution to carbon emissions is to stop using any substance that emits it when burned. Use your imagination. Or maybe anthropogenic global warming models were just wrong all along, or maybe the effects of warming aren't all that bad. $\endgroup$
    – jpmc26
    Jun 8, 2018 at 9:28
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    $\begingroup$ @PinkSweetener You speak as if we can only sustainably produce methane from cows. Not true, in a clean energy future we could synthesize methane with the Sabatier process, with the perk that it feeds off atmospheric CO2. $\endgroup$
    – Saiboogu
    Jun 8, 2018 at 12:43
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    $\begingroup$ Methane is vastly easier to store and transport than hydrogen. And hydrocarbons overall offer ease of transport and storage unmatched by many other energy storage mediums available today or imagined in our near future. Plus chemical energy storage is the only viable option still for certain scenarios, such as space launch vehicles. $\endgroup$
    – Saiboogu
    Jun 8, 2018 at 18:24
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It's unlikely that they would be repurposed. Since natural gas is the least carbon-emitting of the fossil fuels, it's likely to continue to be used for longer than coal, and its use is likely to tail off over a period of fifty years or more.

The key thing here is that pipelines don't stay in good shape without maintenance and as natural gas usage ramps down (and remember it hasn't even peaked yet) it's likely that the system will be abandoned a bit at a time when the cost of operation and maintenance exceeds the value of using it.

The long-distance pipelines have significant redundancy and as production drops, the redundancy will decrease as specific pipelines can no longer earn their keep and are abandoned. The local distribution system has no redundancy, so as usage declines a section of pipes serving a part of a city will earn less and less and at some point it will be more economical to transition the last uses to whatever the new power sources are and stop maintaining the pipes.

(Paragraph added based on questions below) The process will be gradual. As usage of natural gas decreases and the cost of extraction increases, people will move to other power sources and the industry will become less and less profitable. The companies will react by cutting costs, including maintenance (which will suffer for years before a pipeline is abandoned.) Another way to cut costs is to cut off local delivery networks that are no longer profitable and to drop entirely redundant long-distance pipelines which are no longer needed. A section needing expensive maintenance or replacement will be an especial trigger to abandon it. The whole system will slowly shrink and in general, the parts which get abandoned will be older and in need of repair.

It's possible that the pipes will be dug up and recovered for raw materials. That's plausible enough, but depends on fiddly legal details (the gas company owns right-of-way access rights, but who has the right to dig across your property to pull up the pipes?) and on relative costs compared with other sources.

But the bottom line is that the pipeline system is likely to be in pretty rickety shape by the time it's all abandoned and not likely to be useful for very much.

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    $\begingroup$ I'm not sure I understand how a single pipeline would be incrementally abandoned rather than suddenly abandoned. One year it will economically feasible to maintain the pipeline, and then in the next it won't. ISOs bid for grid capacity three years ahead. I think it's entirely plausible that a pipeline could be suddenly abandoned if regional natural gas suppliers obtain an insufficient number of CSOs in the Forward Capacity Market to justify keeping everything running full speed. $\endgroup$
    – Pink Sweetener
    Jun 7, 2018 at 16:56
  • $\begingroup$ @Pink Sweetener: I don't understand what your point is, but I've added some more material to the answer which may help. $\endgroup$
    – Mark Olson
    Jun 7, 2018 at 17:15
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    $\begingroup$ @Pink Sweetener: They said that about railroads, also. And about the bridges and tunnels and subways of New York City. When a industry is failing and losing money, maintenance always suffers. (So does pretty much everything else, too, of course.) $\endgroup$
    – Mark Olson
    Jun 7, 2018 at 19:10
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    $\begingroup$ @MarkOlson Yes, but to the extent that the pipelines couldn't be used for anything else? A pipe that was yesterday DOT-approved to transport explosive gas will probably be good enough to serve as a water slide tomorrow. Maintenance issues for these things typically relate to preventing gas leaks (a hard task that we often fail at) - I have trouble imagining a situation in which pipes are abandoned in such poor condition that they can't even keep some wires safe from tree roots. $\endgroup$
    – Pink Sweetener
    Jun 7, 2018 at 19:43
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    $\begingroup$ Upstate New York has a really lovely network of bike paths built over repurposed railroad pathways and bridges, a great example of repurposed infrastructure. My beloved NYC Subway absolutely needs some work, but even if it became twice as decrepit and ended up shutting down, some of the tunnels could be used for other things. Subterranean subway station restaurants? Who knows. I just don't see your basis for declaring that no pipelines will be abandoned in a condition that will allow them to be reused for any purpose. $\endgroup$
    – Pink Sweetener
    Jun 7, 2018 at 19:47
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Many misinformed Answers here

People should actually read the references they have quoted.

Not an answer, just a point of view from a Natural Gas / Petroleum Engineer

The answers about pipelines will still be used for 'biofuels', 'biogas', 'landfill gas', 'Hydrogern' storing CO2 etc are very misinformed. Those Wikipedia pages are misinformed as well, they are 'theoretically true' The truth is, compressing is expensive.

These pipelines have utility for Natural Gas since NG is extracted from deep, high pressure high temperature wells (HPHT wells), a natural high pressure is provided by the reservoir, which let's us humans to use/transport the gas at a reasonable cost. The cost of compressing increases exponentially as the difference between input and output pressure rises, compressors increase the pressure from 10-20bar (natural) to 30-100bar for long distance transmission only. These gaseous mixtures do not have the natural pressure that NG has, an oil company could compress a gas from 15bar to 30bar; for lower initial pressure than that, the owner would rather burn it (flaring), than find a use for it. In Fact, any gas is useless for commercial applications if it has no pressure. To think these pipelines can be used for used biofuels is naive, it's like traveling on an Autobahn with a bicycle. Answers here are talking about compressing any gas like it's free.

As someone who has been around pipelines and gases all my life, I would say the accepted answer is plausible.

enter image description here

This is from one of those studies that answerers here just saw the title of, read the conclusion. enter image description here

EDIT

There is a lot of discussion on using alternate energy careers and fuels in these pipelines, ignoring the thermodynamics and mechanics of such operations. Hence, I am posting a few sentences from my favorite undergrad book (Mccabe-Smith-Harriot 5th Ed, Page 204) to make a few things clear about why these pipelines, are natural gas pipelines.

*The theoretical head developed by a centrifugal pump (context: any pressure head increasing device), depends on the impeller speed, the radius of impeller and the velocity of the fluid leaving the impeller. If these factors are constant, the developed head is same for fluids of all densities and is same for liquids and gases. The increase in pressure, however, is the product of the developed head and the fluid density. If a pump develops a head of 100 ft and is full of water, the increase in pressure is 100*62.3/144 = 2.9 atm. If the the pump is full of air at ordinary density the pressure increase is about 0.007atm.*

Natural gas is at a high pressure naturally, rising up due to its own pressure from thousands of feet under the earth. It also has a higher density due to high pressure (see here). Hence, the pressure difference created by a compressor on such an input is large. Once you lose pressure of Natural Gas, it is effectively wasted (because the cost of compressing is high), hence flared (unless the volume is very large).

So when the answers above discuss biogas, landfill gas, biofuels, hydrogen, CO2 to use in these magnificently long and large diameter pipelines; it's naive, because there are no known high pressure, significant volume sources of these gases. And unfortunately, it also exhibits an ignorance of the properties of these alternate fuels that are being promoted. These fuels are compressed to burn, and not transport at the scale at which a country requires. Compressed air is used in many industrial operations, but not as a primary energy carrier. You don't want to put in more energy, than you get to use later.

Using these pipelines for such purposes is not the way forward. These gases may be mixed with natural gas for transport, but that is also silly; because you manufactured the gas after investing in separation processes, and then you mix it back with another gas to transport. Plus, the question implies scenarios when we run out of natural gas.

On a side note, I do not think the Earth running out of natural gas anytime soon.

These are actual pressures in pipelines. In a gas distribution network, notice entry point is high, Those are big pipelines where most of the pipeline money went. The city is just the delivery point, a tiny part of the network.

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  • $\begingroup$ I wish I could pick your answer as well. $\endgroup$
    – Pink Sweetener
    Jun 8, 2018 at 16:46
  • $\begingroup$ What do you think about the below answer from @mart on Compressed Air Energy Storage? Not compressing gas for transport, but rather for use as energy storage seems to comply better with the information you provide above, and it's something already done at some renewable facilities so there's at least some evidence that it's cost effective. But are the pipes designed for this? Could air in long tubes double as storage and short-range transmission? $\endgroup$
    – Pink Sweetener
    Jun 8, 2018 at 16:48
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    $\begingroup$ Balderdash. One cubic meter of natural gas is about ten kWh. A typical small workshop compressor can deliver 30 m^3 per hour at six bars and takes 4 kWh. That's 13 percent. The numbers are a bit different for air, and we need a bit more than six bars, but this is surely not out of the world. Oil producers flare the gas because the investment for a gas pipeline can earn more money elsewhere. Legislation could force them, and it wouldn't make a big dent in anyones pocket. $\endgroup$
    – Karl
    Jun 9, 2018 at 11:59
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    $\begingroup$ Ok, 1) Karl's comment is for pressures at around 6bar pressures, please apply the bernoulli's equation and figure out how far the gas can travel at that pressure. And let me know if a gas can traverse long distance pipelines at that pressure. I will then ask my boss to use your theory. Btw, You can buy a $ 500 compressor that will give you that output, we'll throw those $ 50k to $ 1mil compressors that we use. $\endgroup$
    – pyeR_biz
    Jun 11, 2018 at 16:04
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    $\begingroup$ 2) You say Sabatier operates at 15 atm, who operates it? Humans, in a plant, using energy they transferred (remember it is heated 300-400C, should you ignore that fact?), even if it comes from by product of solar power; it is still costly because you had to build that solar panel and related materials to transfer that energy. I may be wrong about how much energy is used up. But the concept is not wrong. Natural gas is available to us at high pressure naturally, its benefits outweigh the cost of drilling it. 3) Again bernoulli, friction factors etc. $\endgroup$
    – pyeR_biz
    Jun 11, 2018 at 16:07
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If you are going to use them, the only thing I can think of is a vacuüm train. Although it's likely not using a "full" vacuüm but a low-density air environment as the pipes wouldn't be able to handle full vacuüm in all likelihood.

What you do is place a small maglev train in them for transport of goods. The lowered air pressure means lower air resistance. The speed and relative silent operation allow you to unburden a portion of the normal tracks and roads.

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  • $\begingroup$ I was going to post something like this. Basically as a kind of hyperloop, at least for the really big ones. This chart shows that most lines are 8" or less, which would not work well for any kind of transportation. $\endgroup$
    – AndyD273
    Jun 7, 2018 at 17:44
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    $\begingroup$ @AndyD273 Maybe a hyperloop transport for really small objects, aka one of those pneumatic tubes at the bank drive-through scaled up to cover inappropriately large distances. Depending on how seriously OP wants to take their economics, I could see a story in which these networks replace Amazon delivery drones. (If the tube diameter is large enough, you could get (cold, stale, nasty) pizza delivery from the other side of the country!) $\endgroup$
    – Ti Strga
    Jun 7, 2018 at 21:54
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    $\begingroup$ It's the Alameda-Weehawken Burrito Tunnel! $\endgroup$
    – Micah
    Jun 8, 2018 at 4:45
  • $\begingroup$ @TiStrga You could actually use it like a train. Have several containers that are the width of the pipe, and a driver either pushing or pulling them through. The 8" pipe has something like 7.5" inner diameter, meaning you could have a 7" cylinder run through it easy, which is pulled (or pushed) by a little motorized engine. It could be a couple feet long probably meaning it would work for a lot of stuff that you'd want to ship. $\endgroup$
    – AndyD273
    Jun 8, 2018 at 14:09
  • $\begingroup$ @Micah I was thinking more on these lines ;) what-if.xkcd.com/31 $\endgroup$
    – Demigan
    Jun 8, 2018 at 17:43
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It's already been done. And its being done a LOT.

Williams Brothers was a pipeline company founded in 1908.
A lot of the nations natural gas pipelines in place had been placed by them. In the 1970's they diversified into several companies, one of them a communications division. This division would use pipelines that had been decommissioned to pull fiber optic cable through the unused lines. You can read in the Wikipedia article how they built a nationwide fiber optic network TWICE. They sold the first one to LDDS (which became Worldcom, then MCI). The second continued to be owned by Williams Communications until they went bankrupt in 2001, where it was acquired by Level 3 communications.

Technology has advanced for having both through the same line concurrently. It is now common practice to pull at least one fiber optic cable through natural gas pipelines that is used (at least) to monitor the pipeline.

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To add to Josh King's excellent answer -

We'll still need natural gas for plastics.

In the US, natural gas is one of the main feedstocks for plastics, beating out coal and oil as the primary source for carbon polymers.

https://www.alleghenyfront.org/this-is-exactly-how-natural-gas-gets-turned-into-plastics/

https://extension.psu.edu/how-plastic-is-made-from-natural-gas

Even if we get to the point where 100% of our plastics are recycled, continued population growth means the market for plastics will outpace the supply of recycled materials. In addition, replacing metals and woods with plastics as products made from those materials wear down will reduce the environmental impact of mining and forestry.

And plastics are used for a lot of things that people consider critical infrastructure. If you watch the videos below, you'll notice that the American Plastics Council focuses on biotech, but it's actually in many things that your day-to-day life depends on.

https://www.youtube.com/watch?v=rE70FAEOX-M

https://www.youtube.com/watch?v=l2Yh_y7Wlbk

https://www.youtube.com/watch?v=viTtllRFYsc

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  • $\begingroup$ Gas will certainly continue to be needed for plastics, but it wouldn't require a very branched network of pipelines, would it? It's not like every producer of a plastic product actually starts out from the raw gas; rather the polymers are prepared at a few big chemical plants. The resulting dense resins / thermoplasts are then transported further by ship, rail or road. If you'd want to use the pipelines for these, you'd have to equip them with conveyor belts running inside, which would be cool but probably wouldn't pay off. $\endgroup$
    – leftaroundabout
    Jun 11, 2018 at 11:32
  • $\begingroup$ @leftaroundabout "It's not like every producer of a plastic product actually starts out from the raw gas; rather the polymers are prepared at a few big chemical plants." That's actually what I'm talking about - the wells where the raw gas is piped up from and the chemical plants are in separate states, sometimes thousands of kilometers apart. We currently use the pipelines to transport the gas from the wells to the plants where it's refined into plastic, so there's no reason to assume we'd stop just because we no longer the gas as fuel. $\endgroup$
    – KernelOfChaos
    Jun 11, 2018 at 23:31
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You would still use them.

It's a post-fossil economy but that doesn't mean that natural gas produced in factories -biogas- (like landfill gas from waste or some kind of algae process of biofuel that if you want I can explain more) wouldn't be used. Natural gas would still be a good energy source, and if it's produced from organic compound it won't increase the net value of CO2 in the atmosphere.

Also, natural gas isn't the only energetic gas, you can still send hydrogen produced artificially with water electrolysis (break water in H2 and O2 that I can also explain) and send it with the pipelines (note that hydrogen made with electrolysis is only an energy vector/carrier because it doesn't have net energy gain, if you look that read biohydrogen).

Finally, I don't know if it's possible to send liquid on that pipelines, but you could send some biofuels.

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  • $\begingroup$ Why the downvote? If you explain me I could improve my answer. $\endgroup$
    – Ender Look
    Jun 10, 2018 at 18:25
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Compressed Air Energy Storage

You post fossile fuel economy would need some energy storage. One way to achieve this is compressed air energy storage: Compress air using solar or wind power, expand it through turbines at a later time to regain some of the energy. The pipelines not only connect points, they themselves also provide a certain volume to work with.

The system would look like this:

  • Places near large pipeline terminals that are well suited for wind or solar power generation compress air into the pipelines
  • This process also creates heat, which will either go to waste or be used in co-located industrial processes or even district heating (though I doubt many compressor sites will be situated in residential areas)
  • At gas terminals, generators with expanders supply electrical power as needed
  • This requires heat, this is could supplied by some renewable source (biogas?)
  • Alternatively, the cooling gases go through when expanding is used in a co-located industry
  • Gas storage tanks, caverns etc. can also be used

I think you can't give a roundtrip efficiency of such a CAES system without accounting for energy use at co-located industry. Something between 50% and 70% could be achievable. That's the thing about a post fossile economy: You have to treat energy (in any form - heat, cold, electricity ...) as if it's worth something. That means energy flows will be more closely coupled.

There are of course headaches:

  • Air needs to travel from compression sites to expander sites. Depending on air volume, there will be pressure losses (=energy losses) along the way. This will effectivly limit how much power can be transmitted like this.
  • Flushing the natural gas from the pipelines will be interesting: Normally you'd flush a pipe from one end with an inert gas like N$_2$ until the natural gas level on the escaping end is safe (Gas on this end will be used until to diluted, then flared, then vented). For flushing the whole pipeline system, cheaper inert gases would have to be used. CO$_2$ maybe (but from what source?) or sometimes water could be used. I'd expect the refurbishing of a gas pipe network to compressed energy storage to employ a generation of engineers & tecnicians.
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Use them as a minable resource.

The Romans built a quite extensive network of aquaeducts. After the fall of the Roman empire, they met all kinds of fates, from continued use to being dismanteled.

The is one Roman aquaeduct in present day Germany, the Eifel Aquaeduct, which was used as a stone quarry. Note, even the limestone deposit within the aqueduct was used.

Depending on the setting of your post fossil-fuel world, gas pipelines could make for a useful "natural" resource. Think, of modern day low background steel, which is a similarly artificial "natural" resource, which is mined for its special properties.

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For the larger pipes, I can see them being re-purposed for long distance shipping of time sensitive items, or just less than 1 day shipping across the country.

Shipping companies might use automated skate boards, luge, bobsleds, etc. to whip through these pipes carrying 1 or more packages. Packages that can be delivered without the need of long-haul semis, being thrown on an airplane, or couriers could reduce the time to cross mountains or areas with few roads.

Shipping this way probably wouldn't eliminate current shipping methods, but could act as a supplementary method for super fast delivery.

However, a break in a pipe, a mechanical failure on a delivery system, or a host of other things could cause massive problems, due to not being able to easily access the tubes for maintenance and clearing blockages. An accidental breakage due to someone digging in the wrong spot or an earthquake could cause all kinds of issues as well.

This idea would assume that the inside of the pipes are (for the most part) seamless, otherwise very smooth, and without quick dips or bumps.

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  • $\begingroup$ This is very close to what I was going to answer, and although not quite, you can add this to your answer: Pneumatic parcel delivery, like a big version of the old school pneumatic tube. $\endgroup$
    – Bohemian
    Jun 11, 2018 at 11:47
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Compressed Hydrogen Gas

Electrical energy can be stored as chemical energy by electrolyzing water. The hydrogen can be transmitted in pipes the same way as natural gas.

The major problem is that hydrogen damages metals, and the mitigations add cost. It's conceivable that existing pipelines could be retrofitted with protective liners.

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  • $\begingroup$ Why the down-vote? $\endgroup$
    – Spencer Joplin
    Jun 10, 2018 at 20:43
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The obvious answer is a massive bagpipe, which the US military could use to globally coordinate ship and troop movements. Such nation-spanning pipes should be able to produce sound heard all the way across both flanking oceans.

If the military by then has better ways of communicating, then I believe storks could use the pipes as a more efficient way of delivering babies. Flying through the air carrying a squirming child has historically been error prone. If enough pipes are available, storks could just put the babies in pipes and then use air pressure to push babies to destinations. I think this might decrease the total number of storks required to do baby delivery, which is good since most bird species have been declining.

Final alternative, pipes could be used for secret storage. By capping one end, people could whisper secrets into the open end... the secrets would then be contained. Passwords, aliases, hideouts-- any secret a person doesn't want to accidentally share could be stored in a nationwide series of secret storage tunnels. By timing the echo length of the pipeline -- which could be days for really long pipes -- people would have timestamps for when to stop by to recover their secret, with an option to whisper it again. People could encrypt it by speaking in their preferred animal language, in case they miss their timestamp. As people outside the same family rarely speak the same animal tongue (no one outside my clan so far as I know speaks bottlenose dolphin, for example), that would protect from most accidental hearings.

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  • $\begingroup$ This made me laugh - the storks are a bit far out, but I do love bagpipes! Obvious, indeed! $\endgroup$
    – bytepusher
    Dec 17, 2019 at 22:52
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They could be converted to a renewable fuel source. Josh King mentioned renewable sources of natural gas, and Spencer Joplin mentioned hydrogen, although most natural-gas pipelines would need to be replaced to carry hydrogen anyway. Other strong possibilities are methanol, which replaces gasoline and can be synthesized by hydrogenation of captured carbon dioxide using hydrogen produced from renewable energy and water; dimethyl ether, a gas which can be produced through dehydration of methanol and can replace either propane or diesel (in redesigned engines); and ethanol, which is more dangerous to store and transport than methanol but can easily be produced from plants in a carbon-neutral, renewable way.

These would not use the last mile of pipe leading to people’s homes, but the accepted answer of running power lines and fiber through them could.

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