Task: Question Generation
Input (English): Passage: As the North Tower collapsed on September 11, 2001, heavy debris hit 7 World Trade Center, damaging the south face of the building and starting fires that continued to burn throughout the afternoon. The collapse also caused damage to the southwest corner between Floors 7 and 17 and on the south face between Floor 44 and the roof; other possible structural damage included a large vertical gash near the center of the south face between Floors 24 and 41. The building was equipped with a sprinkler system, but had many single-point vulnerabilities for failure: the sprinkler system required manual initiation of the electrical fire pumps, rather than being a fully automatic system; the floor-level controls had a single connection to the sprinkler water riser; and the sprinkler system required some power for the fire pump to deliver water. Additionally, water pressure was low, with little or no water to feed sprinklers.After the North Tower collapsed, some firefighters entered 7 World Trade Center to search the building. They attempted to extinguish small pockets of fire, but low water pressure hindered their efforts. Over the course of the day, fires burned out of control on several floors of 7 World Trade Center, the flames visible on the east side of the building. During the afternoon, the fire was also seen on floors 6–10, 13–14, 19–22, and 29–30. In particular, the fires on floors 7 through 9 and 11 through 13 continued to burn out of control during the afternoon. At approximately 2:00 pm, firefighters noticed a bulge in the southwest corner of 7 World Trade Center between the 10th and 13th floors, a sign that the building was unstable and might collapse. During the afternoon, firefighters also heard creaking sounds coming from the building. Around 3:30 pm, FDNY Chief Daniel A. Nigro decided to halt rescue operations, surface removal, and searches along the surface of the debris near 7 World Trade Center and evacuate the area due to concerns for the safety of personnel. The fire expanded the girders of the building, causing some to lose their structural integrity. This led column number 79, a critical column supporting a large part of the 13th floor, to buckle, causing the floors above it to collapse to the fifth floor; however, this could not be seen from outside the building. The structure also developed cracks in the facade just before the entire building started to fall. According to FEMA, this collapse started at 5:20:33 pm EDT when the east mechanical penthouse started crumbling. Differing times are given as to what time the building completely collapsed: at 5:21:10 pm EDT according to FEMA, and at 5:20:52 pm EDT according to NIST. There were no casualties associated with the collapse. NIST found no evidence to support conspiracy theories such as the collapse being the result of explosives; it found that a combination of factors including physical damage, fire and the building's unusual construction set off a chain-reaction collapse.
Output (English): What was a sign that firefighters noticed that meant the building might collapse?