Off Brazil’s northeastern coast, where the sediment-heavy water of the vast Amazon River tips out into the Atlantic, are two very different types of treasure. The first is an ecological gem: a 3,600 square-mile deepwater coral reef discovered less than a decade ago. The second treasure puts the first in immediate danger. Billions of barrels of oil may lie in the ancient sediments beneath the seabed, and licenses have just been approved to drill there.

A few hundred miles north, off the coast of Guyana, companies are already pumping around 650,000 barrels of oil a day from a huge deep-water reservoir discovered in 2015. The find has transformed this rainforest-carpeted country into the planet’s newest petrostate and highest oil producer per capita.

Several thousand miles inland to the s

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