Towering flames illuminated the pre-dawn darkness, casting shadows on the ship Ocean Intervention III as it floated over the sunken remains of the Deepwater Horizon oil rig. The resonant hum of helicopters fused with the roar of fires on either side of the ship, and Chris Reddy could feel the heat on his face.

The night of June 21, 2010, Reddy and colleagues from the Woods Hole Oceanographic Institution were whisked off their research vessel Endeavor to collect samples directly from the blown Macondo well, which had been spewing oil and natural gas into the Gulf of Mexico for two months. They had 12 hours to do something that had never been done before: Use a robot arm to stick a special bottle directly into the hot hydrocarbons. Now, a year later, their analysis explains just what came out of the well, and sheds more light on what happened to it.

It turns out that certain chemicals in the well behave differently under high pressure than they do at the surface. This explains why some chemicals, but not others, made their way into the huge 22-mile plume of oil that Reddy et. al uncovered last summer. It also explains why some scientific papers examining the spill have seemed to contradict each other, according to Don Rice, director of the National Science Foundation’s chemical oceanography program.

“We now have a far better understanding of how and why an oil ‘spill’ into the ocean from below differs from one from above. The significance of this work extends well beyond the Gulf of Mexico,” he said in a statement.