WASHINGTON (AP) - Scientists are reporting the first conclusive evidence of a deep underwater oil plume from the BP spill in the Gulf of Mexico. But don't go looking for it. It's invisible.

A team from Woods Hole Oceanographic Institution in Massachusetts mapped a 22-mile-long mist of oil with special instruments that look for the chemical signature of oil. Scientists say the most troubling thing is that it's not degrading very fast. That means it could be a long-lasting threat for marine life deep in the ocean.

The work is published online Thursday in the journal Science.