Viewed from above, the spot where the world's largest river meets the sea looks like an Impressionist painting. Satellite images of the mouth of the Amazon show brick-red tendrils of water spilling into the deep blue of the Atlantic. The sediment-laden freshwater billows and plumes in the salty ocean.

But up close, this place is muddy and opaque, the water darkened by sediments picked up during the river's nearly 4,000-mile journey across the continent. It's an unlikely home for coral — a famously fragile form of life. So when a Brazilian colleague told oceanographer Patricia Yager that he wanted to investigate a decades-old report that coral reefs were flourishing in the chaotic landscape, “I kind of looked at him like he was crazy," Yager told the Los Angeles Times.

"You know how muddy it is there — how could there possibly be a reef there?” she recalled thinking.

Nevertheless, Yager, a marine science professor at the University of Georgia, helped the fellow researcher, Rodrigo Moura of the Federal University of Rio de Janeiro, get the equipment he needed to dredge the sea floor. What he pulled up astonished them both: vibrant corals, brittle stars, sponges, spiny lobsters and an array of fish. The find, reported Friday in the journal Science Advances, covers about 3,700 square miles off the coasts of French Guiana and northern Brazil — all where scientists had assumed corals just couldn't grow.

“This is something totally new and different from what is present in any other part of the globe,” Fabiano Thompson, an oceanographer at the Federal University in Rio, told Smithsonian magazine. “But until now, it’s been almost completely overlooked.”

The find comes at a time when reefs worldwide are under immense stress. Last week, a task force in Australia reported that 93 percent of the Great Barrier Reef has suffered at least some bleaching. The warming of oceans because of climate change and El Niño have weakened the world's coral for months, scientists say, causing damage from Hawaii to the Indian Ocean.

But the ecosystem at the mouth of the Amazon seemed to be doing well, despite the muddy water there blocking much sunlight from reaching the reef. In the northern part of the plume, where the sea floor is shielded from sunlight much of the year, sponges and carnivorous creatures dominated. In the sunnier south, colorful corals — which depend on a symbiotic relationship with photosynthetic algae and reef-building organisms called polyps — became more common.

All told, the researchers in Brazil found 61 species of sponge, 73 types of fish, 35 species of algae, 26 soft corals, 12 stony corals and more. Dozens of those are thought to be previously undiscovered species.

Their existence demonstrates that reefs can thrive even in suboptimal conditions, Moura and his colleagues wrote. Although the Brazilian reef is less diverse than the vast, colorful cities of ocean life that spring up in the crystalline waters of a place such as Australia or Belize, it is still a successful ecosystem. Studying it could give insight into how corals survive when circumstances aren't in their favor.

But the mouth of the Amazon is likely to become even less hospitable to sun-seeking marine life, the researchers note.

“In the past decade, a total of 80 exploratory blocks have been acquired for oil drilling in the study region, 20 of which are already producing,” the study authors wrote. “... Such large-scale industrial activities present a major environmental challenge."

High levels of fishing in the area also pose a threat to the reef, Yager told the L.A. Times. And since environmental-impact assessments were conducted before the reef was discovered, it's not clear how much damage drilling and fishing might cause.

“Isn’t that always the case?" she said. "You don’t know what you’ve got until it’s gone.”

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