BP Plc engineers using undersea robots had a massive metal chamber hovering just above a gushing, ruptured oil well in the Gulf of Mexico on Saturday in a mission seen as the best chance yet to contain what could be the most damaging U.S. oil spill.

The 98-ton structure has been lowered to the seabed almost 1 mile below the surface. The mission requires pinpoint accuracy in the dark and under high water pressure.

The container was suspended just over the leak while crews using remotely operated vehicles prepared the seabed, said the Unified Command Center, which is coordinating spill-fighting efforts.

“It will hover there until they are ready. They hope to lower to sea floor today, but they need to finish prepping the surface,” the center said in an update late on Friday.