A 100-ton containment box has been hauled out to the Gulf oil rig to divert the thousands of gallons of crude oil that continues to leak from it.
British Petroleum said the dome has been lowered nearly a mile beneath the water. An undersea robot will then position it over one of the two main oil leaks.
Experts say the box has to be accurately positioned or it could damage the leaking pipe and make the problem even worse.
This type of operation has never been tried at that depth in the ocean. If all goes as planned, the dome should collect as much as 85 percent of the oil and funnel it up to a tanker.
Officials said major progress has been made getting the dome in place on Friday. If all goes well, the device will be fully operational by Sunday.
"We haven't done this before," said BP spokesman David Nicholas. "It's very complex and we can't guarantee it."