City officials believe the sinkhole outside Ms. Lewis's apartment may have been caused partly by excessive weight on unstable ground.

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The unstable nature of the ground dictated that houses were no higher than two storeys.

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Campaigners are worried the building work could lead to the ground becoming unstable.

BBC: Concern over homes to be built above Bristol caves

With buildings were still unstable, with fires raging below ground of 2, 000 degrees or more, there was good reason for their concern.

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Unstable means that if an imaginary balloon filled with air from near the ground were to be lifted upward, colliding with some weather feature such as a cold front, the "balloon" would become warmer than the surrounding air at that level.

CNN: The making of a nightmare tornado

Water from the swollen River Exe has washed away the ground from underneath some of the nearby track, buckling rails and making the area unstable.

BBC: South West rail problems could last five days