Sean Gardner / Reuters

A worker takes soil samples on an island in Barataria Bay to determine if the island needs to be cleaned again near Myrtle Grove, La., on March 31. Oil fouled Louisiana's coast in the wake of the wake of the Deepwater Horizon drilling-rig explosion on April 20 last year.

One year ago this week, an oil-rig explosion in the Gulf of Mexico touched off a deep-sea leak amounting to 2.5 million gallons of Louisiana light crude every day for months. In all, nearly 207 million gallons (4.9 million barrels) of oil are thought to have gushed from the leak, along with huge volumes of methane. So what's happened to all those petrochemicals over the past year? The answer is surprisingly complex and contentious.

Or maybe it shouldn't be so surprising. After all, the task requires figuring out what effect Mother Nature and millions of gallons of dispersants had on the plumes of oil and gas, as much as a mile beneath the sea's surface. What's more, the question carries policy implications: BP and the other companies that operated the well would have an interest in downplaying the spill's long-term legacy, while that's exactly the issue that BP's critics want to highlight.

The legal implications could also be huge. Scientists already are finding that their studies are being impeded by civil and criminal investigations into the spill and its effect. For instance, researchers looking into a spate of dolphin deaths that may be linked to oil-fouled seas were told by the National Marine Fisheries Service to keep mum about their findings. "Because of the seriousness of the legal case, no data or findings may be released, presented or discussed outside the UME [unusual mortality event] investigative team without prior approval," the agency told scientists in a letter.

Even the federal government's assessment of what happened to the oil, released last August and updated in November, has been widely criticized by experts who think it downplays the seriousness of the spill's impact. Georgia Tech biologist Joseph Montoya complained last year that the federal government's estimates "always seemed to be biased to the best case."

But here are a few statements that everyone can agree with: Some of the oil evaporated, some was gobbled up by microbes, some was burned, some washed up onto shore, some is still washing up as tar balls, some was dispersed in the sea, and some settled to the bottom of the ocean.

Most researchers also agree that the spill was a catastrophe, no matter how the percentages for those various categories add up. "This was an ecological disaster, no doubt about it," Terry Hazen, a microbial ecologist at the Lawrence Berkeley National Laboratory, told me.

Can Mother Nature clean it up?
A study led by Hazen and published in the journal Science last August lines up on one side of the oil-spill controversy: He and his colleagues reported that a newly identified bacterial strain was digesting the oil at a faster-than-expected rate. "We took 170 samples from where the plume was and couldn't detect any oil in the water column," Hazen said. The researchers also saw no sign of oxygen depletion, which often arises as the result of microbial blooms.

Hazen said only 6 percent of his team's deep-sea core samples contained oil contamination that could be associated with the spill. Additional oil washed up on shorelines and sank into the soil, Hazen said, but he said it may be riskier to do "aggressive treatment" of that soil than to leave it alone.

"Nature does a pretty good job of cleaning herself up, and we shouldn't be mucking things up unless we know what we're doing," Hazen said.

He said that 400,000 barrels' worth of oil (1.7 million gallons) leaks into the Gulf of Mexico from natural seeps every year, and that the Gulf's ecosystem has evolved to handle such natural contamination. "This has been going on for millions of years, literally," he said. "The bacteria that degrade oil are naturally adapted to degrade this oil. They do it quite well."

Mucked up at the bottom of the sea
Studies conducted by the University of Georgia's Samantha Joye and her colleagues tell a different tale: During diving expeditions on the Alvin submersible vessel, they found that areas of the seafloor around the spill site were covered with an oily muck and littered with dead organisms.

So how does Joye answer the "where's the oil" question? "A lot of it's on the bottom, and it's on the bottom all over the place," she told me. "The question is, how long does it stay on the bottom?"

Joye said her findings don't really contradict Hazen's. She stressed that the results from his team on microbial digestion were based on the degradation of a particular component of the oil known as alkane, in a particular zone of the Gulf waters. "His results were based on the deep-water plume, and some people have extrapolated that to the entire oil spill," she said. "And I think that's inappropriate."

She said the Deepwater Horizon blowout of 60,000 barrels a day dwarfed the natural seepage of 500 to 1,000 barrels a day, and doubted that "magic microbes" could have made much of a dent in last year's spillage.

Hazen acknowledged that the area around the spill site is all mucked up, but says his analysis of core samples led him to a different conclusion. He pointed out that during one phase of the response to the spill, millions of gallons of heavy drilling mud were pumped down into the well in an unsuccessful attempt to perform a "top kill" and stop the leak.

"We can see the oil there, but we can also see aluminates and silicates and clay," he told me. "What we're seeing in that layer close to the wellhead is oil that was trapped in the drilling mud."

The bottom line
For now, the best that Hazen, Joye and other researchers can do is agree with the federal government's estimate that roughly a quarter of the oil that leaked from the Deepwater Horizon well was captured or burned at the surface, and then keep trying to track down what happened to the other three-quarters. The federal estimate suggests that a little more than half of the oil has dispersed, evaporated or dissolved. That would leave a little less than a quarter as "residual" oil — that is, oil that looks like oil.

Joye thinks the federal estimate is too optimistic. "The majority of that stuff is still in the system and on the seabed," she said. But gathering the evidence to back up that view will take months or years — which is generally the way it works in science, especially when what you're studying is a mile deep.

"We have to evaluate and very carefully monitor the system to see how long it takes to recover," Joye told me, "because I don't think we can even begin to predict the recovery trajectory at this point."

How quickly will the Gulf recover? What do you think? Feel free to weigh in below with your comments as well as your pointers to other perspectives.

Extra credit: Last August's report from the U.S. Geological Survey and the National Oceanic and Atmospheric Administration estimated that nearly 207 million gallons of oil leaked from the broken well, with nearly 35 million gallons' worth collected by a temporary containment cap. That implies that a little more than 172 million gallons actually leaked into the Gulf. Caveat: The federal report says there's a 10 percent uncertainty factor to its numbers, and as I've tried to make clear above, some researchers don't trust the federal figures.

More about the Gulf spill anniversary:

• A year on, Gulf still grapples with oil spill
• Survey suggests Gulf is nearly back to normal
• Interactive: Sizing up the spill's impact
• Along the Gulf, the spill defines a state of mind
• Interactive: The physics of oil spills
• The latest news and videos about the oil spill

Join the Cosmic Log community by clicking the "like" button on our Facebook page or by following msnbc.com science editor Alan Boyle as b0yle on Twitter. To learn more about my book on Pluto and the search for planets, check out the website for "The Case for Pluto."