While passengers sit comfortably in an airliner’s cabin enjoying their inflight entertainment systems and meals served onboard (at least on international flights), myriad other activities are going on below the floor under their feet. Starting at the nose of the aircraft, a maze of computers and electronic equipment is housed in the avionics bays located just below the cockpit. Immediately aft of that is the wheel well that holds the retracted nose landing gear inflight, and then the cabin pressurization equipment.
Multiple air-conditioning “packs” are ahead of the forward baggage bay filled with luggage and cargo which is packed in large bins on wide-body aircraft. In the center of the lower fuselage is the wing spar carry-through, sometimes fitted with a center fuel tank on long-range aircraft. Behind the wing box are the main landing gear wells, and then the aft baggage bays. Lavatory service equipment is located in both the forward and aft lower fuselage, and finally the APU or Auxiliary Power Unit is mounted in the tail.