This week, two Florida State University oceanography professors dug trenches on a stretch of Pensacola Beach that had recently been cleaned of visible oil and tar balls. What they found, reports National Geographic, was unsettling: Large swaths of oil up to 2 feet deep remained.

“So far, we haven’t seen any rapid degradation in these deep layers,” said Professor Markus Huettel, although he noted that oil at the top of the sand has been disappearing within days.

Huettel and colleague Joel Kostka are biological oceanographers who have been studying the effectiveness of microbes at breaking down oil from the Deepwater Horizon spill. They said that whether microbes munch the oil — the most common way oil breaks down — depends on how much oxygen is available for the tiny organisms to do their work. Unfortunately, much of that oil gets trapped underground when tiny oil droplets penetrate porous sand or when waves deposit tar balls and then cover them with sand.