On May 26th BP, the company responsible for the well that Deepwater Horizon had drilled, started to pump heavy drilling mud into the well's blowout preventer, a set of valves on the sea floor.
The key is the drilling and management of several dozen boreholes around London to pump out about 70m litres (15m gallons) of water a day, equivalent to about 3% of Londoners' daily consumption.