Deep-water corals keep good records, which come in handy in the case of the BP Deepwater Horizon oil disaster. Researchers from Penn State University discovered this week that the impact of the BP oil disaster on corals living in the cold waters at the Gulf of Mexico seafloor is bigger than predicted.

This study joins dozens of others on fish, dolphins and birds as part of the ongoing Natural Resource Damage Assessment, a legal process that’s critical for tracking the damage that started four years ago at the bottom of the Gulf. Scientists first discovered corals coated in a brown substance only 7 miles from the now-defunct BP well in late 2010. The oil left over from the disaster is more difficult to find in the deep sea (in contrast to the coastline, where the occasional 1,000-pound tar mat washes up on shore), so scientists must look to corals for clues on how the marine environment was impacted. “One of the keys to coral’s usefulness as an indicator species is that the coral skeleton retains evidence of the damage long after the oil that caused the damage is gone,” said lead researcher Charles Fisher.

As you can see in the photo above, the normally gold-colored coral has a number of patchy brown growths, which is not found on healthy coral colonies. This coral has been damaged by BP oil.

So how did the oil get so far away from the source? Since these corals are deeper and further away than those previously discovered, Fisher said it could mean that the oil plume could have been bigger than we thought. Potentially, more oil sank to the seafloor than scientists originally predicted.

Not surprising, BP is already trying to refute the scientists’ work, claiming that the corals could have been oiled by the oil and gas that naturally seep up through the Gulf seafloor. However, natural seeps release only 40,000 gallons a day through small cracks in the seafloor across the entire Gulf of Mexico, from Cuba to Mexico to Mississippi. BP released seven times that—2.5 million gallons a day—in one part of the vast Gulf. It seemed obvious that so much oil over a concentrated area of the seafloor would have serious impacts on our deep-sea corals, and after years of careful study, researchers are now providing the scientific links to document those injuries.