The commotion started at the edge of the lagoon.

A man in a blue T shirt began shouting at the families frolicking in the shallows, beckoning them to the shore, urging them to leave the water. We’d seen the same man earlier, helping a swimmer to safety through crashing Atlantic surf. A lifeguard, tour guide or restaurant owner, he clearly knew his patch.

After a morning visiting the Sapiranga Reserve – a secondary Atlantic rainforest in Bahia, Brazil – we were relaxing over a beer and plate of grilled fish on the beach. Intrigued by the commotion, we joined the crowd gathering at the water’s edge. Perhaps an anaconda had slithered down from a cashew tree, an alligator had emerged from mangrove roots, or a sloth had traversed the zipwire.

But the danger was far more insidious.

On the seaward side, rolling sand dunes slope steeply into Atlantic surf. A stranded purple jellyfish shimmered on the fine, golden sand.

Creamy froth bubbled where the fresh water from the lagoon met alkaline seawater and precipitated its cargo of dissolved organics, bubbling into the sand.

We’d been swimming in waterfalls a few hours earlier. The same river cascaded down to the sea, trapped by windblown sand dunes to form a long lagoon where little silver fish darted in the shallows.

One minute the sands of Imbassai were pristine.

The next minute the shallows were alive with droplets of oil. Little gobbets, about the size of a medal, wobbled on the beach like polished jet-black stones. Writhing fat slugs of crude oil sailed in on the ripples to join them.

Where had the oil come from?

A cargo ship perhaps?

There are almost 10,000 oil tankers on the sea, about 10% of the world’s shipping is ferrying oil, or its products, from one place to another. A single super-tanker can carry 2 million barrels (84 million US gallons, 300,000 m3). Most oil spills come from shipping accidents – collisions and groundings. There were three major releases in 2018 – two off the coast of China and one in the Persian Gulf. But there has been no accident reported in the Southern Atlantic.

An accident on an offshore oil rig?

Brazil is the 12th largest producer of oil in the world. Its current output is more than a supertanker’s worth of oil per day, with plans to triple production over the next decade by extracting oil and gas from 6 km below the surface of the South Atlantic sea. The pré-sal oil fields lie under 2 km of water and 2 km of sediment sitting on top of a 2km layer of salt. To get under it requires some ambitious new drilling technology.

Can Petrobras, the nationalised oil producer, mired in corruption cases and financial scandals, be relied upon to tell the truth?

Or has the oil spill, washing ashore on the coats of Brazil, come from further afield? The president, Jair Bolsonaro, has already pointed the finger at Venezuela. The right-wing, business-friendly populist Brazilian president does not have the strongest environmental credentials right now, but Venezuela has the largest oil reserves in the world and is a country in chaos.

What is clear is that there has been a serious accident somewhere.

With a full understanding of what has happened, the size of the spill, it might be possible to limit the damage to marine life and protect the tourist industry on which much of Bahia depends.

Someone knows what happened.

Fiona Erskine’s debut thriller “The Chemical Detective” is published by the Oneworld imprint, Point Blank Books, available in all good bookshops and online here.