Nasa released a picture of what wiring under space shuttle Payload Bay looks like, which was picked up by the redditor piponwa. This is something really unbelievable.
He further writes This is not the actual payload bay of a Space Shuttle, but the Shuttle Avionics Integration Laboratory (SAIL) at the Johnson Space Center. The laboratory is a skeletal avionics version of the shuttle that uses actual orbiter hardware and flight software. The facility even carries the official orbiter designation as Orbiter Vehicle 095.
Payload is the carrying capacity of an aircraft or launch vehicle, usually measured in terms of weight. Depending on the nature of the flight or mission, the payload of a vehicle may include cargo, passengers, flight crew, munitions, scientific instruments or experiments, or other equipment. Extra fuel, when optionally carried, is also considered part of the payload. In a commercial context (i.e., an airline or air freight carrier), payload may refer only to revenue-generating cargo or paying passengers.
For a rocket, the payload can be a satellite, space probe, or spacecraft carrying humans, animals, or cargo. For a ballistic missile, the payload is one or more warheads and related systems; the total weight of these systems is referred to as the throw-weight. (via wikipedia)