Most people would probably vote sharks as their number one fear when thinking about swimming in the sea.

Here’s the thing though. The amount of shark attacks on humans per year globally is a tiny fraction compared to the number of people who drown because they get swept up in a strong current.

Most of us have experienced the joy of swimming in the sea and being flung around by incoming waves. That’s part of the fun. But it can also be dangerous.

While the wind accounts for most of the waves on the surface of the ocean, other things, such as the gravitational pull of the moon, the density of different sections of water mixing together and the salinity (salt content) of the water all affect the under-surface currents which can be far stronger.

The main culprit, believe it or not, is salt. That’s right, that very same shiny, innocent-looking, tasty condiment we sprinkle on our food everyday.

Why? Because different bodies of water in the oceans contain different levels of salinity. When they collide, the denser water (the one with the most salt content) starts to flow under the less dense water creating a current where the water with less salt (think of a clear jug of water mixing with a jug of water with a few cups of salt) rises to the top of the mix. Now expand this to the oceanic scale and you can start to see how dangerous this can be for anyone (humans that is, some animals and fish thrive on it for feeding) trapped within this vortex trying to swim against this force.

So if you want to avoid currents, stick to the pool!