Done in unmodded Fallout New Vegas for PC. Use the fan found in the locked room and put it on the fallen piece of ceiling behind the front desk. Jump off the fan and onto the floor above. From here you are free to explore the room, as well as a void outside the door. If you walk above the garbage can in the inaccessible bathroom and jump off the ledge while staying close to the wall, you can now swim below the building.
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I've been unable to reach the second floor. How was it done? I'd post a "needs verification...