A huge 3,600 sq mile (9,300 sq km) coral reef system has been found below the muddy waters off the mouth of the river Amazon, astonishing scientists, governments and oil companies who have started to explore on top of it.

The existence of the 600-mile long reef, which ranges from about 30-120m deep and stretches from French Guiana to Brazil’s Maranhão state, was not suspected because many of the world’s great rivers produce major gaps in reef systems where no corals grow.

In addition, there was little previous evidence because corals mostly thrive in clear, sunlit, salt water, and the equatorial waters near the mouth of the Amazon are some of the muddiest in the world, with vast quantities of sediment washed thousands of miles down the river and swept hundreds of miles out to sea. 

But the reef appears to be thriving below the freshwater “plume”, or outflow, of the Amazon. Compared to many other reefs, the scientists say in a paper in Science Advances on Friday, it is is relatively “impoverished”. Nevertheless, they found over 60 species of sponges, 73 species of fish, spiny lobsters, stars and much other reef life. 

Its discovery came as a complete surprise, says co-author Patricia Yager, a professor of oceanography and climate change at the University of Georgia. “I was flabbergasted, as were the rest of the 30 oceanographers. Traditionally, our understanding of reefs has focused on tropical shallow coral reefs which harbour biodiversity that rivals tropical rainforests,” she told The Atlantic.

But the reef, no sooner found, is said to be in grave danger. According to the paper, the Brazilian government has sold 80 blocks for oil exploration and drilling at the mouth of the Amazon and 20 of these are already producing oil – some, it is thought, right on top of the reef.

“These [exploration] blocks will soon be producing oil in close proximity to the reefs, but the environmental baseline compiled by the companies and the Brazilian government is ... largely based on sparse museum specimens. Such large-scale industrial activities present a major environmental challenge,” said the study’s authors.

The Amazon is the world’s greatest river, collecting water from an area over 7m sq km