A hot springs bathhouse in the Underworld district beneath Milwaukee, heated by the geothermal energy that gives the Underworld its name. The springs are natural — or at least as natural as anything gets in a subterranean district built in repurposed mining tunnels — fed by groundwater that passes through geothermally heated rock at depth and surfaces in pools that maintain a constant 40 degrees Celsius. The bathhouse was built around these pools by an Underworld construction crew who recognized a commercial opportunity when they struck hot water instead of ore, and the result is a series of interconnected bathing chambers carved from living rock, lit by thermal-powered lanterns, and operated with a simplicity that borders on ancient.

Magma charges 15 Quanta for unlimited access and provides nothing but hot water, clean towels, and silence. There are no screens, no feeds, no music, and no talking in the main pools — this last rule is enforced by staff who will remove violators with a gentleness that somehow makes it worse. The clientele is a mix of Underworld residents for whom the baths are a daily routine, surface workers who descend specifically for the hot springs, and a surprising number of off-duty military contractors who claim the mineral water helps with the chronic pain that comes from their profession. The water tastes of sulfur and iron, and long-term regular bathers develop a mineral sheen on their skin that is either a health benefit or a very slow form of petrification, depending on who you ask.