BP says its engineers have successfully lowered a containment box over a well that has leaked millions of litres of oil into the Gulf of Mexico.
The complex operation to drop the 90-tonne chamber about 1500m below the surface is expected to be completed on Sunday.
BP hopes the four-storey structure will capture oil from the largest leak so it can be funnelled to waiting ships.
The company hopes the 100 tonne device will be operational by Monday. It may take several days before it becomes clear whether the dome will work.
Oil has been leaking unstopped for 18 days from the well, 80km off Louisiana, since an explosion destroyed the Deepwater Horizon rig, killing 11 workers. It sank on 22 April.
The delicate operation is seen as the best hope to stave off the biggest US environmental disaster since the 1989 Exxon Valdez spill in Alaska, but officials remain cautious about whether it will work.
Oil has begun washing ashore on the Chandeleur islands off Louisiana, which are part of the Breton National Wildlife Refuge.