It has to be. When an aircraft is pessurised, the pressure has to be even, or compensated for by strong matterial. This is why aircraft are basically cylindrical, with pressure bulkheads at either end. If the underfloor hold was not pressurised, the floor would need to be very strong to cope with the load placed on it by internal air pressure.
A practical example of what happens when this situation is suddenly changed is the THY DC-10 crash in Paris in the '70s. The cargo door was incorrectly designed. As the aircraft was climbing, it blew out. The underfloor hold instantly depressurised, but the passanger section had no way to do so, meaning there was a very sudden and rather high pressure differential between the area above the floor and under it. As a result, the rear cabin floor collapsed into the hold and fell out the back of the aircraft, taking many passangers withit. This also severed all the control cables, and the aircraft entered a dive and crashed, killing all on board.
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