The mouth of the Amazon River. (Photo: NASA/Public domain)

In their most iconic form, coral reefs are found in crystal-clear tropical waters, not the muddy mouth of a river. In fact, Patricia Yager, a oceanographer at the University of Georgia “kind of chuckled” when a Brazilian colleague, Rodrigo Moura, suggested they search for a reef in the mouth of the Amazon, The Atlantic reports.

When their team found evidence of a reef there, she told the magazine, “I was flabbergasted, as were the rest of the 30 oceanographers.”

The reef they found is more than 600 miles long and covers 3,600 square miles, they describe in a study published in Science Advances. In parts, it’s covered by the silty outflow from the Amazon River only three months of the year, so that the corals’ photosynthetic symbiotes can photosynthesize at will. In large parts of the reef, though, sunlight is harder to come by, and those areas are populated by sponges and carnivorous fish.

The scientists did have a clue that they might find a something of interest in this area: Moura had found a paper from 1977 in which fish indicating the presence of a reef had been found in the area. On a research cruise, the team dredged the ocean floor and came up with coral, sponges and reef fish. They had found exactly what they were looking for.*

*UPDATE 2/2/17: A scientist with the California Academy of Sciences explains how this wasn’t exactly a new discovery.

Bonus finds: A 200-year-old pub with bottles of brandy still intact

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