The Lake Under the Stairs is a new Hearth & Horizon story (text only), set in an invented priory on the Mosel in 2047. The arithmetic behind it:
- Fermenting must at 215 g/L sugar gives about 100 g of CO2 per litre, so 3,000 L make about 300 kg, roughly 165 m³ at room temperature. The cellar is 144 m³.
- Gas heavier than air settles. At 1.7 m³ an hour (a quarter of the total in the busiest day, an assumption) over 48 m² of floor, the surface rises at most 3.6 cm an hour, reaching knee-height valves in about 11 hours and a wall sensor at 1.5 m after about 43.
- So a sensor at head height can read 0.52% while the floor reads 3.4%. Holding the working-day limit (5,000 ppm) takes about 370 m³/h of extraction at floor level.
Real cellar deaths are why the story ends on a two-person rule and not a candle. Read it on the Stories page. The figures are kitchen-table estimates, not safety advice.