Five years ago this month, the BP Deepwater Horizon drilling rig exploded in the Gulf of Mexico, causing a blowout of its oil well. By the time the well was capped nearly three months later, some 4 million gallons of oil had spewed into the water, according to the latest estimates from independent scientists, creating the largest oil spill ever in U.S. waters. (Legally, BP will be held responsible for 3.19 million barrels, according to a January ruling by a federal district judge*.)

In the weeks after the explosion, Smithsonian sent reporter Michelle Nijhuis to Louisiana, where she found few tourists, lots of oil and tar and teams of scientists documenting the effects of the spill and trying to find ways to clean up the mess. At the time, fisheries scientist James Cowan and environmental biologist Ralph Portier, both of Louisiana State University, were just beginning to study the oil-dredged coast. This week, Smithsonian caught up with the pair and asked them to reflect on what happened and what they have learned from the disaster.

The following has been edited for length:

When Nijhuis visited the Louisiana coast in 2010, she saw “tar balls as big as manhole covers” and “oily sheens, some hundreds of yards across.” What does the Gulf look like today?

JC: One of the things you have to know about Louisiana is that spills as large as the BP spill are infrequent, but smaller spills occur all the time. Today, there might be sheens and oil floating on the surface that have nothing to do with Deepwater Horizon nor approaching the magnitude of what was visible in 2010. But I can tell you that there are places, particularly in some of the estuaries, where you can go up on the marsh with a shovel and still collect oil in a bucket without spending much time working. So there still appears to be a lot of oil in Louisiana estuarine systems in some places.

RP: We’re hoping we dodged a bullet. This was not an oil spill; it was a continuous oiling event. We had repeated waves of oil coming ashore. A lot of that oil fortunately degraded. But there may be a suspended layer of oil at depth in the water column and there’s also marginally weathered, degraded oil sitting on the floor of the Gulf. We still have mats of oil coming ashore, and because we can fingerprint that oil, we know its oil from the BP spill.

What kind of changes did you see after 2010?

JC: One of the acute things that we started seeing soon after the well was capped—beginning in November 2011 and on into 2012—were fishes with sores and lesions on them. Most of the fishes with sores were red and vermilion snapper, but we observed sores on more than 25 species. Eventually, a group at the University of South Florida linked the spill directly to those lesions and sores.

We had been working offshore prior to Deepwater Horizon because we realized that there was almost no baseline data on many of our coastal habitats; this led us to begin sampling the offshore reefs on the edge of the Louisiana continental shelf before the spill occurred. Fortunately, the reefs west of the mouth of the Mississippi were unaffected, which was a relief to me because they’re beautiful, diverse and productive. For the most part, the acute impacts have passed, although oysters have been very, very slow to recover. And it’s unclear whether they ever will.

In the weeks after the spill, Portier expressed hope that he would be able to get permission to try to clean up some of the oil via bioremediation—using bacteria that could munch on the hydrocarbons and fertilizers to promote bacterial growth. What became of that effort?

RP: We did some studies evaluating biological products that could be used in an oil spill scenario. And these products were already on an EPA-approved list. We were told, however, that we needed to reevaluate these approved products on this particular oil, which we did at LSU. But field trials were rejected. It was argued that in the Gulf of Mexico’s warm climate, the situation would clean itself up. I think over the past five years, the argument can be made that that’s not exactly what happened.

We’re still finding oil from the Exxon Valdez spill in Alaska, more than 25 years later. For how long will we continue to find oil from the Deepwater Horizon spill?

JC: We just don’t know what to expect. If there is oil remaining on the seafloor that was covered by sediments from a very large 2011 river flood, it may have sequestered in areas where weathering has been slowed by low oxygen. There is worry that when we do have a big tropical storm, oil that has been buried or sequestered might become remobilized.

RP: When we clean up smaller spills here along the Gulf coast, we can see the faint residue profiles of old oil spills from previous years. You can see the layers. You’re not cleaning up one spill; you’re cleaning up four or five. So you will always see some legacy that an analysis will say, yes, this was from the 2010 event.

There have been many scientists studying the Gulf since the spill. What do we still need to know?

JC: We don’t know very much about the coastal ocean and the coastal habitats it supports. Right now, if something bad happens, we are not able to triage the western Gulf of Mexico on the continental shelf because we don’t have baseline data. So we couldn’t tell you what is normal or abnormal. We probably know more about some of the reefs in the Indo-Pacific than those in our own backyard.

RP: One of our challenges is knowing when a spill is “over." That’s important because if we have another spill, we need to identify the culprit. But can we now say that the BP spill is over, because the impacts on fisheries, wetlands, key marine mammal species and bird populations have apparently ended? Not yet. We are still lacking definitive data sets showing life cycle or next generation recoveries from oil exposure in these key sentinel species. But that day is approaching, we hope.

It sounds like we’re not all that prepared for another spill like this in the future.

RP: There are technologies that probably should be strongly evaluated as tools—pumps, skimmers, even ships that work as oil-water separators. Those things should be known entities and part of a game plan to deal with a future spill, because we have quite a few platforms in the Gulf of Mexico. We also need to learn a lot more from our colleagues in Europe. They have in place the technology that would have prevented a significant amount of this oil escaping. The problem in the Gulf of Mexico became a more difficult one simply because of the sheer volumes of oil involved. If we had had that equivalent amount of oil spilled here in south Louisiana as was spilled by the Exxon Valdez in Alaska, we wouldn’t be talking about it five years later, wondering if we still had hidden ghosts from the BP event.

*Update: This story has been amended to reflect the most recent estimates of the amount of oil released during the spill, as well as the amount for which BP will be held legally accountable.