You could maybe domesticate moles?
Yes, but we’d be farming like that now if it were possible. Best bet might be some crop that is toxic until processed -bitter cassava for example.
How about a system whereby the survivors make foraging trips to the surface and gather… basically, whatever organic materials they possibly can. They then take these materials underground, and use them in the most efficient possible manner, whatever that might be, to provide a food supply for the colony.
I’m thinking specifically about using the ant colony farming methods nature has proven viable here. The ants carry leaves underground, which they compost and use to grow fungi, which they eat.
Humans might, let’s say for example, have an underground farm of some sort. Animals include cows, chickens, and pigs. Cows eat grass, chickens eat grain, pigs eat pretty much anything. This would provide a basic diet of milk, eggs, and meat.
Light is minimal, so… Let’s start there. Hydroponics. Obviously there’s not enough power to grow everything belowground, so reserve the hydroponic systems to low-mass-high-value plants. Herbs and spices, medicinal plants, and ‘luxury’ goods when possible, ie: tomatoes and such. Nutrients for the plant will come from processing of the colony’s wastes, water supply (grey water possibly), and whatever electric sources you might let your colony have.
More food is needed, so let’s work in the fungus. Bonus is that you don’t need the electric lighting, but you still need water and nutrients. You can grow a LOT of mushrooms off just a couple cubic meters of ‘soil’, and the nice part of this is that the soil will be constantly resupplied by the wastes from the rest of the colony. Some of these mushrooms feed the colonists; the rest go to feeding the farm animals. I’m sure pigs will eat mushrooms quite happily; chickens might? Don’t think cows would, but I’m a city boy so I’m guessing here anyway. Perhaps there are other valuable food animals that can be fed primarily by mushrooms?
Pigs eat mushrooms, mushrooms eat pigshit. Stable loop? Not quite. Thanks to entropy, you need to add something here that grows in sunlight, which is the point that’s been made over and over previously in this thread. That’s where the surface-gatherers come in.
You say it’s possible to occasionally visit the surface, and if so it’s possible to gather something usable on each trip. Ideally this would be things like gasoline and venison, but don’t forget about the massive resources that exist in the form of wild plants, even those not directly edible by humans. Make a rule whereby each person returning to the colony has to bring in SOMEthing organic, if they don’t have anything more valuable this can be leafy branches from nearby trees, or just a few bucketfuls of nice loamy soil, preferably with earthworms. Of course, you’d want fruit and vegetables when available, so depending on the situation small orchards and gardens could be planted. Perhaps there are berry bushes nearby as well, or a fish pond, river, lake, or even the ocean. Basically, if there’s any environment better than a desert on the surface, you’ve got a potential source of raw materials. In a survival situation, you’d do well to get the most value possible from it.
I think, depending on the ratio of underground survivors to foragers, a stable or at least semi-stable colony could survive this way. You’re still getting most of your energy from sunlight, but all the real ‘farming’ is taking place underground, converting the most basic of resources through your farm animals into usable people-food, in whatever the most efficient manner might be. I envision the cycle looking something like grass-fed to cow-turned to cheese-turned to poop-fed to mushrooms-fed to pigs-turned to bacon-turned to poop-turned to mushrooms-turned to pigs-turned to bacon, and so on until all the valuable nutrients are used up, at which point the depleted ‘soil’ you end up with goes into an incinerator to generate electricity, and the ashes buried somewhere. Or something like that.
The nice part of this setup is that the balance is key. The more efficient the imput of raw materials, the more efficient the recycling system underground, the less foraging that needs to be done on the surface. If the colony can forage a lot, then it can grow a lot. If they can barely meet their needs, you’ve got a balance. If they can’t quite sustain this for the long-term, you’ve got an excellent source of tension with which to spur adventure on the surface for your characters. If they can’t sustain it anymore at all, then you have an exodus story or a nice depressing end-of-it-all story.
Who knows how long a system like this could last?
Significant nitpick: it’s been proven viable at that scale, but I suspect it would not be viable for humans due to a number of factors that increase cubically rather than linearly (load carrying capacity per individual, energy requirement to move about, etc)
Also, mushrooms just aren’t a wonderful source of nutrition. 100g of mushrooms contains about 40 calories. An active adult male would have to eat about 6 kilos of mushrooms a day (which I think would cause digestive problems for various reasons).
And mushrooms generally don’t live on shit. Can we just put that misconception to bed? They generally feed on decaying organic matter - cultivated mushrooms are grown on composted stable manure - comprising mostly sawdust or straw - animal dung provides some important nutrients for both the mushrooms and the other organisms that break down the straw, but mushrooms don’t live on shit (well, some species do, but not the ones you’re interested in farming).
How about growing potatoes (or similar) on the surface, utilizing the sunlight and low maintenance aspects, but harvesting them from below ground, with either some careful shallow mining, or brutal methods along the lines of preparing an underground pit below the crop, then collapsing the crop into it, caving in the ground above, but leaving the crop accessible for below ground retrieval?..
The trouble with “occasionally grabbing anything organic” is that people have to eat every day. So, every day (on average) every human would have to bring in enough organic material to grow enough food to live on for that day.
This is not easy, which is why subsistence farming is backbreaking constant work. If it was easy to grab a handful of leaves and dirt and grow mushrooms on it, that’s what peasants would be doing instead of growing wheat, rice, or corn.
So if you’re going to the surface once a month, you have to bring back on that trip enough to last you for a whole month, as well as enough to support for a month the other people who weren’t part of the gathering party. This is a lot of food.
Even if there was a gigantic field of wheat growing outside the cave, how much could you bring back if you sneaked out one night? If you could drive a combine out into the field at night it might be a lot. If you’re out there with a sickle trying to gather wheat ears by hand, not very much.
For occasional surface harvesting - how about sunflower seeds?
They’re pretty nutrient dense - 204 calories / 35 grams / 1/4 cup. So 1 days worth of food is only weights about 350 grams and takes up 2.5 cups - not bad for carrying. Plus they’re pretty healthy in general. http://www.whfoods.com/genpage.php?tname=foodspice&dbid=57
As far as growing them without much work - well, they’re basically a weed.
Is there a sunflower seed vault up on the surface?
That cite is for the kernels, not the whole seeds. If there’s a field of sunflowers on the surface, you have to harvest the flowers, separate the seeds, then crack the shells to get the kernels.
Food crops generally are only harvestable for a few weeks out of the year. This is why farmers plant, then tend, then harvest the crops rather than every day wander out into the field, gather enough food for the day, then go home.
You have to realize that what is being proposed is a system of farming where the farmers must live in caves, and can only come out to plow, plant, tend, and harvest the crops a few nights out of the year. How much food do you imagine farmers could produce this way?
Subsistence farming means year-round endless backbreaking labor caring for your tools, your land, your crops, and your animals. A system where you sneak out in the spring, scatter a few seeds, then sneak out in the fall to harvest the crops isn’t going to produce enough food.
Well, not with that attitude it isn’t 
Seriously though, I didn’t say it was a great idea - but surely, if you had to use that system, some foods would be better than others. I proposed sunflower seeds as that - better than other options, without necessarily commenting on the plausability of the idea in general.
I also think it depends on the size of the group we’re talking about. For an underground city - not gonna happen. For 20 or 30 people, some occasional ground harvesting combined with some other ideas could be somewhat workable.
Can they raid canned goods from nearby abandoned supermarkets?
I don’t know how else you’re going to do it, unless you have a huge underground cavern, plenty of soil and water, plenty of grow lights, and some kind of nuclear batteries.
I’m starting to wonder if this is one of those times where story ideas fail upon contact with reality. sigh Then again, given that the idea itself is largely the result of ‘‘Metro 2033’’ (the novel, not the game) cross-pollinating with some urban exploration photos of a really big and rather habitable looking underground limestone quarry in Derbyshire perhaps I shouldn’t be suprised. However, I’ve got a good plot idea so I’m not willing to give up on it yet. More thought is clearly required.
A scenario that might work is a network of caves in a landscape of limestone sink-holes. (We don’t really have this in the UK though). You could have some kind of surviving fauna falling into the sink-holes and becoming available as food, plus very limited growing space for crops on ledges inside the open sink-holes.
Or maybe a very large population of bats - the bats fly out to feed on insects, the underground humans eat Hoi Sin Crispy Bat, with a side of roaches and centipedes (which live in the bat guano)
Whatever your scenario, it needs a net energy input that significantly exceeds the dietary requirements of the humans in the caves.
Mangetout,
I have to wonder if they wouldn’t eventually suffer from something like scurvy. Woulds bats and insects provide all necessary nutrients?
That doesn’t seem like a very complete diet - it’s probably OK in terms of proteins, fats, minerals and fat soluble vitamins, but it looks light on carbohydrates, fibre and other stuff such as vitamin C, folic acid, etc.
Malnutrition would be a risk - probably significantly reduced if just one more food type (a green leafy one) was added.
Upon reflection the scenerio posed by Weaver reminds me of the Ulgos in David Edding’s Belgariad fantasy series. To recap, the Ulgos were people who lived in a region inhabited by “monsters”- all the gods’ half-baked ideas that didn’t work out, and which were berserkly inimical to human life. The Ulgos lived in a network of caverns only emerging to tend their fields at night, ready to run for cover at a moments notice and armed with vicious serrated knives.
Dammit. I hate it when it turns out that I’ve been beaten to what I thought was an original idea.
I’ve done a bit more research on the site of my chosen setting (pictures of which can be found here if you’re interested) and it looks pretty good on the above ground resources front. There’s farmland (not the best but still..), woodland close to the entrances (of which there are three, although I’ve only located one), and a reservoir stocked with trout nearby (although due to the aforementioned not knowing where the entrances are I’m not sure just how nearby). Currently things look rather promising.
Would that be possible? Is compost even flammable?