HOUSTON (AP) -- As engineers bore deeper into the seafloor toward the source of the oil still spewing into the Gulf of Mexico, BP PLC is growing more confident that the relief well it expects to complete in August will succeed where all previous efforts to contain or kill the gusher have failed. But what if it doesn't work? At the very least, oil would continue to spill while workers try something else. That proposition would surely bring more misery for the people who live, work and play along the shores from Louisiana to Florida. And consider this: Chief Executive Tony Hayward said in June that the reservoir of oil is believed to hold about 2.1 billion gallons of oil.