Cleaning up decades-long oil pollution and restoring environmental health in just one of Nigeria’s crude-producing states will cost at least $12 billion, investigators said on Tuesday.

Bayelsa state, home to some two million people, “is in the grip of a human and environmental catastrophe of devastating proportions,” they warned in a much-awaited report.

Lying in the Niger Delta region, Bayelsa is where oil was first discovered in Africa in the 1950s, and where companies Shell and Eni have operated for decades.

“Once home to one of the largest mangrove forests on the planet, rich in ecological diversity and value, the region is now one of the most polluted places on Earth,” the report said.

“At least $12 billion” is needed to “clean up the soil and drinking water, reduce the health risk to people and restore mangrove forests essential to stopping floods.”

The four-year investigation was carried out by the Bayelsa State Oil and Environmental Commission — an international panel of experts and prominent figures who worked at the request of the local government.