London is the capital of the United Kingdom. It has about seven million people. The river Thames runs through London. People from all over the world visit London to see Buckingham Palace, the Houses of Parliament, Westminster Abbey and the Tower of London. There are many museums, theatres and parks in London, as well as offices and factories. London began as a Roman settlement called Londinium. The plague  came to London in the 1600s, followed by the Great Fire of 1666. The city was badly bombed in World War II. London is very slowly sinking into its foundations and the level of the river Thames is slowly rising. As a result, extra-high tides could flood a large part of London. To prevent this happening, a great barrier has been built across the Thames at Woolwich. If very high tides happen, the barrier can be raised and London will be safe. According to the passage, from 1600 to now,   _   great disasters have happened in London.
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