Gulf's Mystery Sheen Baffles BP, Coast Guard But the once-burst well is still sealed By Newser Editors and Wire Services Posted Dec 19, 2012 11:45 AM CST 27 comments Comments Oil from the BP Deepwater Horizon spill floats on the waters off the coast of Louisiana. (AP Photo/Charlie Riedel) (Newser) – Underwater inspections conducted with robot submarines at the site of BP's Deepwater Horizon rig disaster have failed to identify the source of a persistent sheen on the surface of the Gulf of Mexico, officials said yesterday. It's not all bad news: The Coast Guard and BP both said the recent inspections confirmed the well that blew out in April 2010 remains secure and isn't leaking oil. Relief wells drilled in 2010 to stop the gusher also were found to be secure during the four-day survey, BP said. The Coast Guard has said the sheen can't be recovered and doesn't pose a risk to the shoreline. However, investigators collected samples of a white, cloudy substance that appeared to be coming from several areas on the overturned rig on the sea floor. Lab tests were planned on the samples of the substance, which isn't believed to be oil. "Next steps are being considered as we await the lab results and more detailed analysis of the video shot during the mission," says a Coast Guard statement. After a sheen was spotted near the site of the blowout in September, workers capped and plugged the steel container that was suspected to be the source. On Nov. 2, however, BP reported another sheen in the same area.