Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The subterranean or underground rivers of London are the tributaries of the River Thames and River Lea that were built over during the growth of the metropolis of London. Since it is difficult to stop water from flowing downhill, the rivers now flow through underground culverts. Many London localities started their existence as small villages along these rivers, and their place names reflect their origin. In recent years, parts of some London rivers have been restored to their previous above-ground state, and in some cases fish have been reintroduced. In June 2008 outline plans to reinstate some underground rivers were published by the office of Mayor of London, and in January 2009 a partnership between the Environment Agency, Natural England and the Greater London Authority set out a strategy for putting this into effect by creating the River Restoration Centre.