A persistent, mysterious “oil sheen” in the Gulf of Mexico near the site of BP’s Deepwater Horizon disaster grew to more than seven-miles long and one-mile wide during a recent stretch of calm seas, based on aerial observations made by a former NASA physicist turned environmental activist.

“We had maybe three or four days (of calm weather) and that’s all it took for the stuff to build up considerably,” Bonny Schumaker, the physicist who now runs the non-profit On Winds of Care, which makes regular flights over regions of the Gulf affected by the 2010 oil spill.