HOUSTON — BP went ahead on Wednesday afternoon with its most ambitious — and potentially most consequential — effort to plug the mile-deep gusher of oil that has been streaming into the Gulf of Mexico for more than a month.

The procedure, known as a top kill, began at 1 p.m., the company said.

It involves pumping thousands of pounds of heavy fluids into a five-story stack of pipes in an effort to clog the well and stop the torrent of oil. BP officials said this method of containing spills had never been tried so far underwater, and that it could take days to determine whether it had succeeded. They cautioned there was no guarantee that the gambit would work.

Here is a live feed (.wmv) from remotely operated vehicles, which is pretty worthless at the moment. CEO Tony Hayward said it “a day or two before we can have certainty that it’s worked," but evidence of failure could arrive much sooner.