When people picture oil in the ocean, they picture a disaster: the Exxon Valdez, Deepwater Horizon, a burning rig. A new satellite analysis says that picture misses most of the problem. SkyTruth, a US nonprofit, estimates that oil slicks cover about 1.58 million square kilometers of ocean surface every year, and that most of it comes from routine discharges nobody reports.

The report was released on Sept. 23 during Climate Week in New York, and Mongabay's coverage this week brought it to a wider audience. It is an advocacy group's analysis, not a peer-reviewed paper, which matters for how to read the headline number.

What SkyTruth Measured

Two numbers sit side by side in the report, and they are very different. Using radar images from the European Space Agency's Sentinel-1 satellites, SkyTruth's Cerulean system verified 3,152 oil slicks between 2023 and 2025, covering about 22,805 square kilometers, roughly the size of New Jersey. That is what was directly seen.

The 1.58 million figure, an area more than twice the size of Texas, is a model. Sentinel-1 does not image every patch of sea often, and slicks fade within hours or days, so SkyTruth adjusted for coverage, for how long slicks stay visible, and for vessel traffic recorded by transponder data. The report says the open ocean, nearly two-thirds of the sea, has to be almost entirely modeled and carries the lowest confidence. An earlier Science study by Dong and colleagues counted 1.51 million square kilometers of cumulative slicks across five years of satellite images, which SkyTruth used as a starting point.

Ships, Mostly

Vessels account for about 81% of the slick area that could be traced to a source. Offshore oil infrastructure accounts for about 18%. Cargo ships and tankers dominate, and SkyTruth says most of this is illegal under MARPOL, the international treaty on pollution from ships, which limits discharged bilge water to 15 parts of oil per million, according to Mongabay.

The fleet figures are striking. Of the world's tankers, 6.23%, or 1,566 of 25,156, were linked to a detected slick, as were 5.54% of cargo vessels, or 2,495 of 45,042. Flags matter too. China had the most linked vessels, 704, but that is only 0.35% of its fleet. Panama had 611, or 5.44% of its fleet. Flags of convenience, including Palau, Panama, Malta, Comoros, Cyprus and the Bahamas, were linked to slicks out of proportion to their size, which SkyTruth says points to weak oversight by the countries that register the ships.

Sixteen vessels appeared to be discharging oil in more than 10% of the satellite passes in which they were seen. Accountability is thin: ownership data was unavailable for more than half of the polluting vessels SkyTruth looked at. A satellite sighting of a slick near a ship is evidence for investigators, not proof of a violation.

Where the Oil Is

The model flags the Red Sea, the Persian Gulf, the Mediterranean, the Black Sea, the waters off West Africa and the seas around Indonesia as the highest-concentration areas. Over 90% of the pollution lies inside national waters, according to Euronews, so this is largely a coastal problem with identifiable regulators.

The most densely slicked economic zones are those of the Democratic Republic of the Congo, the Republic of the Congo, Bulgaria, Romania and Syria, which are 10 to nearly 40 times more polluted than the national-waters average. In the two Congos, oil infrastructure drives much of it: more than 80% of the slick area in the DRC and about half in the Republic of the Congo. Among directly observed slicks, Indonesia led with 823, ahead of Romania with 245 and Malaysia with 214. A single facility off Indonesia was associated with slicks in about 28% of the images that included it.

Indonesia is already in the news for its worst fires in 11 years, which suggests how stretched its environmental enforcement is on both land and sea. The Persian Gulf and Red Sea hotspots are the same waters at stake in the stalemate over the Strait of Hormuz, where tanker traffic is both an economic lifeline and a pollution source.

Protected Areas Are Not Safe

About three-quarters of marine protected areas had lower slick concentrations than the waters around them, Euronews reported, which is a point in their favor. They are far from clean, though. SkyTruth hand-labeled 27 slicks reaching Conkouati-Douli National Park in the Republic of the Congo, all originating from three structures outside its borders. In a Mediterranean whale and sea turtle corridor off Spain, it labeled 45 slicks, all from vessels, with 20 different ships identified.

The Mediterranean makes up less than 1% of the global ocean surface but about 15% of maritime trade, and three of the 10 most polluted Important Marine Mammal Areas are there. The Gulf of Mexico is the worst, home to Rice's whale, of which SkyTruth says only about 51 remain. It blames offshore platforms and expects the pollution to worsen after a US decision to waive endangered species protections there.

The Gulf of Guinea shows what is at stake. Researchers recently found a living coral reef off Benin, in the same region where SkyTruth counts 162 slicks around neighboring Nigeria's platforms.

The Blind Spot

The report's most useful finding may be about knowledge. SkyTruth says limited satellite coverage leaves vast areas effectively unwatched, so oil pollution can go undetected and unpunished. John Amos, SkyTruth's chief executive, told Mongabay that this kind of pollution was largely invisible before satellite imagery.

My view: the exact figure will be argued over, and it should be. The honest reading is that illegal discharge at sea is common, geographically predictable and rarely prosecuted, and that better radar coverage plus public ownership records would change that more than any new treaty. The test will be whether any flag state or port authority acts on the repeat-polluter list in the coming months.