From unexplored trenches to the Bermuda Triangle, the world’s oceans are filled with unsolved mysteries.

But one of the strangest questions is why a vast patch of the Atlantic Ocean has suddenly begun to cool at record speeds.

Until March, the central Atlantic had been experiencing its hottest warm weather event since 1982, hitting highs of 30° (86°F).

However, this was followed by a dramatic temperature swing, with surface water temperatures plummeting below 25°C (72°F) – and scientists still don’t know what has caused it.

Michael McPhaden, of the National Oceanic and Atmospheric Administration (NOAA) told Live Science: ‘We are still scratching our heads as to what’s actually happening.’

Scientists are currently intensely monitoring the strip of water, which stretches a few degrees on either side of the equator between Brazil and the coast of East Africa.

It is not just that this region – known as the central equatorial Atlantic – is cooling down which is unusual, but rather the speed of the change.

Each year the central equatorial Atlantic swings through its temperature cycle, reaching its warmest points in March and April before cooling down into the summer.

However, starting from June, the surface water temperatures began to fall at an unprecedented rate.

By Mid-June, temperatures in this region were at 0.5–1.0°C (0.9–1.8°F) colder than average for this time of the year.

Water temperatures have now begun to rise back toward the usual levels but scientists have been left baffled by what caused the sudden cooling in the first place.

Dr McPhaden says: ‘It could be some transient feature that has developed from processes that we don’t quite understand.’

Generally, cooler summer waters in the Atlantic are associated with stronger trade winds blowing over the equator.

These strong winds sweep away warmer surface water colder, deep ocean water to rise to the surface in a process called equatorial upwelling.

However, the winds in the rapidly cooling region were actually weaker this year than normal – usually a sign that warmer waters are on their way.

There were some intense winds in early May which might have kickstarted the cooling process but Dr McPhaden points out that they ‘haven’t increased as much as the temperature has dropped.’

Dr McPhaden says that while human-caused climate change can’t be ruled out as the cause it is unlikely to be responsible.

‘At first blush, this is just a natural variation of the climate system over the equatorial Atlantic,’ says Dr McPhaden.

A swing of half a degree on either side of the average might not seem like a reason for much fuss, but scientists are keeping a careful watch over this region of the ocean.