BP is almost ready to attempt what may be the best hope at containing the oil spill in the Gulf of Mexico. The company is building a four-story, 100-ton metal container that it will try to lower onto the leaking well. Once in place, the oil will fill the chamber; oil will then have to be pumped from the chamber into a tanker on the surface. If successful, the chamber could reduce the oil flow by more than 80 percent. However, success is far from given: The source of the leak is 5,000 feet deep, much deeper than the leaks in previous uses of such chambers.