A homeowner builds a personal underground shelter beneath their property, modeled loosely on the sealed civil defence designs used during prolonged sheltering events. The structural sealing is solid, the air supply is filtered. What's missing is the part that matters most during an actual extended stay: a way to remove the CO2 that builds up the moment the hatch closes and four family members...

A naval shore installation seals its underground command facility for a 48-hour readiness drill. No external breach, no contamination event — just a closed-loop airtight environment running on internal life support alone. Twenty hours in, the air feels different. Not visibly, not by smell — but the people inside are sluggish, slower to respond, harder to keep alert on watch. This is...