While the oil didn't seem to have as big of an impact as originally feared, some researchers say it is still affecting the wildlife in the Gulf of Mexico.
Scientists working for the federal government still don't know how much damage the BP spill caused, meaning they don't know how much BP will be fined.
There's practically no visible oil anymore on the Gulf or in marshes, but there are signs that it was there.
Among them, nearly 400 dead dolphins washed up on Gulf shorelines.
Last summer, Louisiana's Barataria Bay was one of the busiest spots on the Gulf. Hundreds of BP's cleanup boats made the bay busier than I-10 at rush hour.
These days, there really isn't anything to block scientist Suzanne Lane's view. She's tracking and photographing bottlenosed dolphins in this bay for the National Oceanic and Atmospheric Administration.
"[I take] kind of like a dolphin census, and then see how the population does throughout the year," Lane explained.
It's all part of a huge effort to figure out just how much damage was caused by BP's record-setting oil spill.
"Dolphins are top-level predators in the estuaries. Looking at them tells you something about how the ecosystem is doing," said Lori Schwacke with NOAA. "What we see in the dolphins is an indicator of what we see in the food web."
The size and health of the population of dolphins that are alive will tell scientists a great deal, but as we near this one year anniversary, they're concerned about the abnormally large number of dolphins that have died.
"We want to know -- A, number one -- why these dolphins are dying. But we also want to know how these dolphins are dying," said Blair Mase, NOAA marine mammal stranding coordinator. "It's such a complex situation because we've never really dealt with anything like this before."
Officials say 375 dead dolphins have washed up on Gulf coast beaches since last February -- six times above average.
At least 12 of the dead dolphins washed up with visible oil on their bodies. The most recent washed up just two weeks ago.
The die-off started before the Deepwater Horizon accident, but spiked dramatically right after the spill and increased again this year.
Scientists want to know what effect the oil is having.
Even more troubling is that many of the dead dolphins were either stillborn or died shortly after birth.
"It is unusual to see that many dolphins -- likely pre-term, aborted fetuses -- washing ashore," Mase said.
Dolphins are pregnant for 12 months, meaning any who died since the spill likely swam through oil & dispersant-filled water at some point while pregnant.
Out on the water a year after the blowout, Deepwater oil may not be visible, but scientists want to know just how much trouble it's still causing dolphins.
"Dolphins can avoid oil; however, the magnitude of this event, you know, it was covering estuaries and bays, and this is their habitat. There was nowhere for them to go," Mase said.
NOAA is testing the blubber of every dolphin that watches ashore. That should tell them what the dolphins died of and how much oil was ingested by the animals.
Bcause that evidence may be needed in court against BP one day, the government won't share any of those results yet.