BP will attempt an unprecedented engineering feat, dangling a metal box the size of a double-decker bus a mile underwater to try to contain the oil leaks responsible for the slick in the Gulf of Mexico. Robots will help to move the canopy into position over the leak. If the operation is succesful, the leaking oil will be siphoned off from the box to an oil storage vessel at the surface. BP said that it could take up to a week to lower the structure into position and bolt it to the seabed. Because of its weight, special feet have been fixed to the structure allowing it to sit on the muddy seabed. About 500 people are now at the site of the deadly accident, 50 miles off the coast of Louisiana, in a flotilla of vessels. The company has declared a partial success in its operation to control the spill after it sealed off one of three leaks from the well pipe by attaching and closing off a valve. However, there are growing fears that the slick could hit the coast of Florida, triggering a mobilisation of National Guard troops as far south as the Florida Keys. US officials said that 7,500 people were now involved in the effort to protect the shoreline of four states from the slick, which measures 130 miles by 75 miles. The Government's weather agency has been warning for days that it could reach the Mississippi Delta, the Chandeleur Islands and the nearby Breton Island. BP admitted that the operation would not reduce the overall rate of flow of the oil, estimated at 5,000 barrels per day, but claimed that it was a positive step because it "reduced the complexity of the situation being dealt with on the seabed". An estimated 2.5 million gallons of crude oil have leaked into the sea since the BP-leased platform sank on 22 April, still burning more than two days after an explosion that killed 11 workers. The Deepwater Horizon leak is more than 15 times deeper than this and lies 4,000ft below the level of the deepest scuba dive.