Their existence has been disputed, but there is now more evidence of oil plumes deep below the water surface:

From the National Oceanic and Atmospheric Administration:
NOAA’s independent analysis of water samples provided from the May 22-28 research mission of the University of South Florida’s R/V Weatherbird II confirmed the presence of very low concentrations of sub-surface oil and PAHs (polycyclic aromatic hydrocarbons) at sampling depths ranging from 50 meters to 1,400 meters.
The samples came from three stations, 40, 45 and 142 nautical miles from the site of the Macondo well.
The hydrocarbon concentration was low: less than 0.5 parts per million, and PAH levels were even lower.
And only one of the three sites contained oil that appeared to come from the BP oil spill; one couldn’t be confirmed and the other may have been from another site. From NOAA:
- Hydrocarbons found in surface samples taken at the Slick 1 source, 40 nautical miles northeast from the well head, were consistent with the BP oil spill source;
- Hydrocarbons found in samples from Station 07—42 nautical miles northeast from the well head—at the surface, at 50 meters and at 400 meters are petroleum-derived but in concentrations too low to confirm the source; and
- Hydrocarbons found in samples taken from Station 01, 142 nautical miles southeast of the well head, at 100 meters and 300 meters were not consistent with the BP oil spill source.