| UPDATED: 08:45, Sun, Feb 14, 2021
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River Thames: RSPCA capture boa constrictor near Barnes Bridge
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The subterranean rivers of London are the tributaries of the River Thames that were built during the growth of the famous city. They flow through underground culverts, with a number of them now integral parts of London's sewage system. The River Fleet is the largest of these, its headwaters are two streams on Hampstead Heath, each of which were dammed into a series of ponds in the 18th century.