In Dwarf Fortress, aquifers are a scourge. They're layers of subterranean, water-bearing rock that continually, endlessly leak when breached by dwarven miners. They can be managed and even exploited with careful construction, but in my experience, their main utility is proving that—no matter how competent at Dwarf Fortress I might get—I'm only one overlooked, unsealed tile away from a disaster I'll only notice once the dwarves in that quarter are already neck-deep in floodwater.
