To date, well operator BP has paid $US42 billion [$A54 billion] in fines, clean-up and compensation costs and the figure is expected to keep growing.
A judge is set to rule soon on additional federal Clean Water Act penalties of up to $13.7 billion, $4 billion of which is payable by minor partner Anadarko.
The BP oil spill was one of the worst recorded in human history, and remains a rallying point for those who call deepwater drilling too dangerous.
The explosion on TransOcean's semisubmersible drilling rig on April 20, 2010, pumped 650 million litres of raw crude into the Gulf of Mexico over 87 days and claimed 11 lives.
Operators on the rig misinterpreted the results of critical safety tests the night before the explosion, failing to notice a cement job had failed, although a faulty blowout preventer was also blamed for the spill.
New rules
The Obama administration has marked the fifth anniversary of the disaster by proposing new regulations aimed at strengthening oversight of offshore oil drilling equipment and ensuring that out-of-control wells can be sealed in an emergency.
The new rules require that blowout preventers in wells have two shear rams which cut through the drill pipe and allow the well to be sealed.
In the Deepwater Horizon spill the single shear ram failed to operate properly.
Two shear rams are now industry standard, but not required.
The rules also require an annual review of maintenance and repair records by government-approved inspectors.
The public has 60 days to comment on the proposal before it is finalised.
Ongoing issues
BP's massive oil spill has continued causing ecological damage such as a die-off of more than 1000 dolphins and is blamed for lesions on other marine species.
Scientists from a Florida-based scientific consortium say the use of the dispersant Corexit in unprecedented volumes left a thick layer of slime on the ocean bottom which has disrupted normal marine life in the gulf.
The same thing happened in 1979 with the explosion and sinking of the Ixtoc-1 rig off the coast of Mexico. The consortium of 13 institutions - including the University of Florida's College of Marine Sciences, Eckerd College and Mote Marine Laboratory in Sarasota — is going back to Ixtoc to assess what the long-term impact of the Deepwater Horizon blow-out will be as part of a $US20 million study.
A study released by Temple University researchers this month found that oil alone wasn't as damaging to the gulf's deep corals, and said oil mixed with Corexit could cause abnormalities in animals and humans.
BP says the results of lab testing are not comparable to conditions in the gulf.
Core samples dug up from the ocean floor in the southern gulf recently still resemble those taken from Ixtoc after the 1979 spill.
More than three decades on they say there is still a thick, dark layer of oil and dispersant stopping small amoeba-like creatures living on the ocean floor, potentially affecting the phytoplankton and zooplankton communities that form the basis of the food chain.
The local shrimp fishery collapsed, but this could have been a result of overfishing rather than the spill itself, the consortium says.
Costs
Louisiana locals say the Lake Pontchartrain Basin, once the engine of the largest oyster industry in the US, has plummeted by almost 70% since the Macondo disaster.
The spill also killed thousands of birds and hundreds of turtles and prompted temporary fishing bans.
BP paid more than $5 billion in compensation to some 62,000 locals who claim they were harmed by the catastrophe. It expects to pay out up to $10 billion.
BP has also committed to pay $500 million over 10 years to support independent research through the Gulf of Mexico Research Initiative. Around $315 million in grants has been awarded to date.
At the end of December last year, BP had spent more than $14 billion and workers had devoted more than 70 million personnel hours on response and clean-up activities.
The US Coast Guard ended the remaining active clean-up operations in the Deepwater Horizon area in April 2014.
If residual oil from the Deepwater Horizon incident is later identified and requires removal, BP will take action at the direction of the Coast Guard.
Criticism
To mark the anniversary Greenpeace, which is fighting Shell's plans for drilling in the Arctic, says the Deepwater Horizon disaster remains a truly shocking episode.
"People everywhere were stunned by the carnage the oil industry had wrought, and how incompetent it was at dealing with the disaster. There was a huge amount of anger," campaigner Ian Duff said.
"BP has suffered financially as a result, but sadly little else seems to have changed. The oil industry is continuing to push into ever more extreme and inhospitable environments, like Shell's plans to drill in the icy waters of the Arctic, where stemming and cleaning up a disaster like this would be impossible. The impact of a spill on the Arctic's fragile ecosystem would be devastating."
