ON THE GULF OF MEXICO — A mission to the bottom of the sea to try to avert a wider environmental disaster progressed early today as crews said a 100-ton concrete-and-steel box was close to being placed over a blown-out well on the Gulf floor in an attempt to capture gushing oil.

Douglas Peake, the first mate of the supply boat that brought the box to the site, confirmed he received a radio transmission from the nearby vessel lowering the device that said the device would be in position over the well soon.

The 40-foot box was about 4,000 feet underwater before dawn today, with another 1,000 feet to go, Coast Guard Petty Officer Shawn Eggert said.

Meanwhile, a separate mission was close to getting under way to spray water around the rig that's drilling a relief well to try to reduce the level of fumes from the thick oil hampering the crew's ability to do its work on deck.

A crane late Thursday lowered the containment vessel designed to collect as much as 85 percent of the oil spewing into the Gulf and funnel it up to a tanker. Eventually the crane would give way to underwater robots that will secure the contraption over the main leak.

A steel pipe will be installed between the top of the box and tanker. If all goes well, the whole structure could be operating by Sunday.