USINSK, Russia – On the tundra outside this oil town near the Arctic Circle, a pitch-black pool of crude stretches toward the horizon. The source: a decommissioned well whose rusty valves ooze with oil, viscous like jam.

This is the face of Russia’s oil country, a sprawling, inhospitable zone that experts say represents the world’s worst ecological oil catastrophe.

Environmentalists estimate at least 1 percent of Russia’s annual oil production, or 5 million tons, is spilled every year. That is equivalent to one Deepwater Horizon-scale leak about every two months.

Crumbling infrastructure and a harsh climate combine to spell disaster in the world’s largest oil producer, responsible for 13 percent of global output. Oil, stubbornly seeping through rusty pipelines and old wells, contaminates soil, kills all plants that grow on it and destroys habitats for mammals and birds.

Half a million tons every year get into rivers that flow into the Arctic Ocean, the government says, upsetting the delicate environmental balance in those waters.

It’s part of a legacy of environmental tragedy that has plagued Russia and the countries of its former Soviet empire for decades, from the nuclear horrors of Chernobyl in Ukraine to lethal chemical waste in the Russian city of Dzerzhinsk and paper mill pollution seeping into Siberia’s Lake Baikal, which holds one-fifth of the world’s supply of fresh water.

Oil spills in Russia are less dramatic than disasters in the Gulf of Mexico or the North Sea, more the result of a drip-drip of leaked crude than a sudden explosion. But they’re more numerous than in any other oil-producing nation, including insurgency-hit Nigeria, and combined they spill far more than anywhere else in the world, scientists say.

“Oil and oil products get spilled literally every day,” said Grigory Barenboim, senior researcher at the Russian Academy of Sciences’ Institute of Water Problems.

No hard figures on the scope of oil spills in Russia are available, but Greenpeace estimates that at least 5 million tons leak every year in a country producing about 500 million tons a year.

Irina Ivshina, of the government-financed Institute of the Environment and Genetics of Microorganisms, supports the 5 million ton estimate, as does the World Wildlife Fund.

The figure is derived from two sources: Russian state-funded research that shows 10 to 15 percent of Russian oil leakage enters rivers; and a 2010 report commissioned by the Natural Resources Ministry that shows nearly 500,000 tons slips into northern Russian rivers every year and flow into the Arctic.

The estimate is considered conservative: The Russian Economic Development Ministry in a 2010 report estimated spills at up to 20 million tons per year.

That astonishing number, for which the ministry offered no elaboration, appears to be based partly on the fact most small leaks in Russia go unreported. Under Russian law, leaks of less than 8 tons are classified only as “incidents” and carry no penalties.

Russian oil spills also elude detection because most happen in the vast swaths of unpopulated tundra and conifer forestin the north, caused either by ruptured pipes or leakage from decommissioned wells.

Weather conditions in most oil provinces are brutal, with temperatures routinely dropping below minus 40 degrees in winter. That makes pipelines brittle and prone to rupture unless they are regularly replaced and their condition monitored.

Asked by The Associated Press to comment, the Natural Resources Ministry and the Energy Ministry said they have no data on oil spills and referred to the other ministry for further inquiries.

Now that Russian companies are moving to the Arctic to tap vast but hard-to-reach oil and gas riches, scientists voice concerns that Russia’s outdated technologies and shoddy safety record make for a potential environmental calamity there.

In 1994, the republic of Komi, where Usinsk lies 40 miles south of the Arctic Circle, became the scene of Russia’s largest oil spill when an estimated 100,000 tons splashed from an aging pipeline. It killed plants and animals, and polluted up to 25 miles of two local rivers, killing thousands of fish. In villages most affected, respiratory diseases rose by some 28 percent in the year following the leak.

Seen from a helicopter, the oil production area is dotted with pitch-black ponds. Fresh leaks are easy to find once you step into the tundra north of Usinsk. To spot a leak, find a dying tree. Fir trees with drooping gray, dry branches look as though scorched by a wildfire. Usinsk spokeswoman Tatyana Khimichuk said the city administration had no powers to influence oil company operations.

“Everything that happens at the oil fields is Lukoil’s responsibility,” she said, referring to Russia’s second-largest oil company, which owns a network of pipelines in the region.

Ivan Blokov, campaign director at Greenpeace Russia, who studies oil spills, said the situation in Komi is replicated across Russia’s oil-producing regions, which stretch from the Black Sea in the southwest to the Chinese border in Russia’s Far East.

“It is happening everywhere,” he said. “It’s typical of any oil field in Russia. The system is old, and it is not being replaced in time by any oil company in the country.”