Between the currently-airing Dungeon Meshi anime and my recent binge of the manga (I finally got past the 40% mark), I've been thinking about the logistics of long-term dungeoneering.
Military theorists and historians have some rules of thumb about what soldiers can be expected to do and need. [citation] For instance, they usually need around three pounds of food per day and can carry around 90-120 pounds of stuff. (The total varies less by strength and more by how much of that strength you can convince soldiers to use carrying stuff the general cares about.)
Theoretically, this means soldiers can carry a month or two of food; however, hardtack makes a pretty terrible weapon. Most of their carrying capacity is taken up by inedible (and also important) gear; the standard rule of thumb seems to be that soldiers can carry about ten days' worth of food.
The same is presumably more or less true for dungeoneers. A wizard's robe, staff, and spellbook probably weigh less than a sword and a suit of armor, but that space is going to get taken up by the miscellaneous tools you need to survive in a dungeon that aren't necessary for armies walking through inhabited lands.
In short, in the absence of Senshi, dungeoneers can only spend about a week and a half in the dungeon. Obviously, you need to set aside time to return to the surface, so you can't go deeper than five days. Well, you can—starvation doesn't kill you instantly—but you really shouldn't.
What if we added some people who only carried food? That would help some. Including two porters per three dungeoneers would roughly double the group's operational endurance, from ten days to twenty.
But the number of porters grows rapidly as the desired trip into the dungeon grows longer; operational endurance to 30 days requires four porters per dungeoneer. Even if the dungeon is spacious for a party of dozens to be possible, having that party be 80% or more noncombatants is a recipe for disaster.
What about pack animals? Mules require about five times as much food as humans (assuming they can't graze in the dungeon), but they can carry close to 300 pounds of supplies. One mule per three dungeoneers extends operational endurance from 10 to 15 days, a second to 17.
That's not bad, but pack animals work better when they can graze. If the dungeon has grass or equivalent foliage, one mule per three dungeoneers increases operational endurance to about 26 days, a second to 35, and one mule per dungeoneer increases it to 39. But most dungeons don't have much to graze on.
For the spendthrift dungeoneer, pack animals have one advantage over porters: You can eat them.
I can't find any actual data about how much meat you get from butchering a mule, but combining other data lets me estimate 300 pounds (with large error bars).
So you could theoretically buy a (relatively) cheap mule at the surface, bring it with you through the dungeon, butcher it when you'd eaten through the supplies on its back, and live off its meat for a while. In this case, you probably don't even need to feed it on the way down! I have been informed that you do, in fact, need to feed it.
Five dungeoneers could live off the supplies carried by an increasingly malnourished mule for about 19 9.5 days. The mule would probably lose weight during that time, but the butcher could probably get at least a hundred pounds of decent meat off the poor critter. That would give them at least a week of extra rations, plus whatever they carried on their own backs, for a total operational endurance of at least five three and a half weeks.
This strategy probably works best if the adventurers are planning to go establish a camp after a few days and linger there for a few weeks. That would let them slaughter the mule as soon as they reach their base camp and free them from somehow carrying a whole mule carcass worth of food around afterwards.
This kind of strategy could enable supply depots relatively close to the surface. If we increase the party from five dungeoneers and a mule to five merchants and twenty mules, they could supply adventurers going a bit deeper. They'd need to charge a pretty hefty surcharge—at the very least, they'd need to cover the cost of killing so many mules!
It's also possible to create supply depots without slaughtering pack animals, but they would need to be smaller, closer to the surface, or both.
It would, strictly speaking, be possible to make a deeper supply depot, supplied by a larger depot. It would probably be impractical, though.
Dungeoneers weighed down by their own equipment can only spend brief periods of time exploring a dungeon. If they include some porters or pack animals in the party, they can increase that to maybe a month (two weeks down, two weeks up).
A sufficiently profitable dungeon economy might enable a set of outposts where adventurers can rest and resupply between treks deeper into the dungeon. If enough pack animals were slaughtered, they might be able to bring supplies a week or two deeper than the surface market.
In the right circumstances, dungeoneers might be able to delve a full month below the sunlit world without eating anything except wheat bread and mule meat. But this requires a small army of merchants and herdsmen and porters and butchers and so on, feeding not just the dungeoneers themselves but all the people supporting them, and all the people supporting those people, reaching through countless miles of cavern and across acres of farmland.
And of course, all of that assumes that no step in this process gets disrupted by the dangers of a dungeon; no wargs killing your mule, no warg packs overrunning the outpost, no getting lost in the twisty little maze of passages all alike. The higher you build that house of cards, the farther you'll far if it fails.
Senshi had the right idea.
