In writing adventure material, nothing pays off like primary research. Look into the lives of rats, and you find that they are purblind and communicate subvocally. Pay attention to stone, and you find that at the juncture of limestone and granite sometimes grows a layer called skarn, laced with gold, copper, and gems. Architecture, chemistry, botany: as much as they constrain design with realism, they also open up intriguing possibilities with the ring of reality to them.
Lack of research also shows. How many lost mines, dwarven or not, have been written up for adventures? How many of them have been glommed together from the residue of Moria-sublime (halls, chasms, demons) and Wild-West-banal (railcarts, lifts, ingots)? The one thing that's certain is that horrible things from the deep have been unleashed and are now running around in the place. But can we do better in setting the scene?
Even cursory research turns up one detail of deep metal mining, in medieval Europe or any other civilization, that presents enormous challenges. Below the water table, mines tend to flood. The simplest solution: dig a drainage channel, or adit, to lower ground. But this presumes your mine sits on higher ground from somewhere. Deep mines don't have this luxury.
So, pump the water out. At first people pass buckets hand to hand, then as craft deepens, machines use hand power, mule power, water power to lift out the groundwater using buckets, screws, suction pipes and tubes. All these latter solutions need keeping up, and once the mine is abandoned, the lower levels partly or fully fill with water.
Flooded floors, concealing pits and swimming monsters; flooded tunnels, requiring magic light and water breathing to have any chance of mere survival. Or, another way: get the old pumping machinery working again and see how much you can clear out, and what treasures lie in the murk.
All this assumes a pre-industrial European level of technology. But a fantasy world also has dwarves, that people of notably precocious craft. Indeed, one solution only they might reach comes from the computer construction game Dwarf Fortress, whose worldbuilding is as complex as its graphics are crude. The game simulates groundwater by having some settlements sit over an aquifer level, whose water floods and ruins all construction beneath it. The way past the aquifer requires one of many complicated engineering solutions, including rapid pumping, opening a shaft to cold air that will freeze the water, or dropping a "plug" of dry stone into the wet level and boring through it.
|Dropping the "plug"
Although Dwarf Fortress simplifies the geological reality of seepage, the plug idea suggests that dwarves might have the skill to locate the source of groundwater and simply wall it off with non-porous stone. Maybe the water is controlled and channeled into a reservoir, for drinking and industrial use.
Allow a certain amount of magic in mining, and the pumping operation can be helped in a dozen ways. Maybe the dwarven priests have deals with elementals, or maybe these solutions are found among other underground peoples, like the dark elves. Golems can be set the task of working the pumps. The miners themselves breathe water in flooded galleries. Magic freezes flooded caverns so that ice tunnels can be dug through. A portable hole, or elemental portal, does the work of an adit in draining off water. And what might come through the portal the wrong way?
Another difference: human metal production historically had to be distributed over several sites, because the material for processing ore -- water, wood, and aboveground oxygen -- was not present within a mine, and not necessarily plentiful close to it. Dwarves, though, live entirely underground. Their mine dungeons necessarily include areas for crushing ore, then sorting and filtering the metal-bearing compounds through the action of water. They need to smelt ore in the heat of a furnace, creating liquid metal. If steel is being made, the fuel needs to infuse the raw iron with carbon. Most likely for dwarves this will be mineral coal rather than the medieval-era charcoal. Why not have the facilities for shaping and working metal objects right there to hand as well? A whole complex suggests itself. The only limit is availability of fresh air and water, which architecture or elementals need to supply. And Dwarf Fortress gives another idea: using the earth's own magma to power fierce furnaces.
In short, thinking about realistic logistics can take you places in design your unconstrained imagination never would. It can insert unforeseen challenges into mundane mines, or underwrite the need for a thematically varied industrial site in the more fantastic variety.