Anaerobic bacteria breaking down organic matter in waterlogged marshes and bogs produce methane (CH₄) as their dominant gas, but trace amounts of phosphine (PH₃) and diphosphane (P₂H₄) can also form from phosphorus-containing organic residues under strongly reducing conditions. Diphosphane is pyrophoric — it ignites spontaneously on contact with air at room temperature, without any spark or flame needed.
A small diphosphane ignition can then set the surrounding methane alight, producing a brief, flickering, low-intensity flame. Because these flames form in scattered pockets of rising gas and drift with air currents near the ground before burning out, this reaction is the leading scientific explanation offered for the "will-o'-the-wisp" or "ignis fatuus" lights long reported over marshes and bogs — though it should be said plainly that not every historical sighting has been independently verified, and some accounts likely have other causes (bioluminescence, reflected light, or simple misperception in the dark).
- Decomposition rate — how fast organic matter breaks down, controlling how often gas pockets form.
- Diphosphane concentration — how much pyrophoric gas each pocket carries; too little never reaches ignition, enough ignites reliably on reaching the surface.
- Air current — the breeze that carries ignited wisps sideways as they flicker and drift.