Science: Undermined City

In Johannesburg a tremor ran through the earth. It shook tall office buildings, cracked walls, swung chandeliers, made restaurant waiters spill the soup. Women screamed, and tourists sprang to the.'r feet asking: "Is it an earthquake?"

Such tremors are not normal earthquakes; they are specialties of Johannesburg. The City of Gold shakes more or less continually because of "rock falls" and "rock bursts" in the great mines under its skyscrapers. It gets from six to ten tremors a day. Most of them are less severe than the worst tremor last fortnight, which registered "four" on the scale of earthquake severity, and was...

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