The day the yellow clams turned black is seared in Ramon Aguero’s memory. It was the summer of 1994. A few days earlier, he had collected a generous haul, 20 buckets of the thin-shelled, cold-water clams, which burrow a foot deep into the sand along a 13-mile stretch of beach near Barra del Chuy, just south of the Brazilian border. Aguero had been digging up these clams since childhood, a livelihood passed on for generations along these shores. But on this day, Aguero, 70, returned to find a disastrous sight: the beach covered in dead clams. “Mile after mile, as far as our eyes could see. All of them dead, rotten, opened up”, he said “They were all black, and had a fetid odour”.

He wept at the sight. The clam die-off was an alarming marker of a new climate era, an early sign of this coastline’s transformation. Scientists now suspect the event was linked to a gigantic blob of warm water extending from the Uruguayan coast far into the South Atlantic, a blob that has only become warmer in the years since.

The mysterious blob covers 130,000 square miles of ocean, an area nearly twice as big as this small country. And it has been heating up extremely rapidly, more than 2C, over the past century, double the global average. At its centre, it’s grown even hotter, warming by as much as 3C, according to one analysis.

The entire ocean is warming, but some parts are changing much faster than others, and the hot spot off Uruguay is one of the fastest. It was first identified by scientists in 2012, but it is still poorly understood and has received virtually no public attention.