In Homer’s Odyssey, the protagonist has to guide his  ship through a treacherous gap.

On one side, Charybdis, a  terrifying whirlpool monster. On the other, Scylla, a six-headed woman who wants to claw out your  eyes then drown you to death.

But although these monsters are fictional, Odysseus’ passage has been  identified with a real place: the Strait of Messina, between the island of Sicily  and the toe of Italy’s boot. The Strait is famous for its treacherous  conditions, strong currents, and yes, whirlpools. But the Strait has another interesting quirk.

The seas on either side of it  have opposite high and low tides. Only a narrow strip of water  separates high tide from low. That might seem impossible.

But it’s all down to a simple quirk of geography. [intro] The Mediterranean Sea, the body of water that separates  southern Europe and North Africa, is divided into a whole bunch of smaller seas, defined by the way they  interact with the coastline. Two of those seas, the Ionian and Tyrrhenian  Seas, both cradle the coast of Italy. The Ionian Sea is underneath the boot, while the Tyrrhenian is to the west of it.

The Strait of Messina, between Italy and Sicily, connects them. Now, besides the aforementioned  mythological significance, these seas have another peculiar property. They’re in tidal opposition to each other, which means that when it’s high tide  in one sea, it’s low tide in the other.

Which sounds a little weird. After all, the two seas are  basically in the same spot. They’re each other’s next door neighbors, which means that the forces driving the tides – the Moon, the Sun, and the rotation of the Earth – should be acting on them in roughly the same ways.

So it doesn’t really make sense for  them to have such different tides. But to break down why this is happening, we first have to zoom way out to  what causes tides in the first place. Imagine Planet Earth without  any land on it at all.

Just a huge, spinning ball covered in water. When the Moon’s gravity pulls on the planet, it’s easy to imagine a section of  that water actually bulging upwards, drawn towards the Moon’s pull. This is called the tidal bulge.

Now, the Earth rotates while the  Moon remains relatively in place, so imagine that the Earth rotates under the bulge and it ends up moving across the entire planet. Right in the middle of the tidal  bulge, sea level would be higher, and the area directly under the  bulge would experience high tide. The size of the tidal bulge isn’t  constant all the time, though.

It’s modulated by the Sun,  which also pulls on the water. That’s why high tides are higher during times when the Moon and the  Sun are pulling in the same direction, and lower when they’re pulling against each other. The Moon and the Sun don’t  create water out of nowhere.

The extra water that goes into the tidal bulge has to come from another part of the ocean. You’d think the water would come  from the far side of the earth, but it doesn’t. There’s a bulge there too, and that’s because the Earth  itself is constantly spinning.

The Earth’s inertia creates a counterweight, a second tidal bulge exactly  opposite to the first tidal bulge. Between them, you have two  areas of lower sea level, which would be considered low tide. So on this water planet, without any continents, any given location would experience  two high tides and two low tides a day.

But the thing is, we do have land, and land makes tides a little more complicated. Except for a few low-lying islands  and the occasional sandbar, the tidal bulge can’t pass over land. That means that that tidal bulge is  constantly crashing into the coastline, and those coastlines can change  the properties of the tide.

This is why in some parts of the world, we can have particularly  dramatic, spectacular high tides, because the coastline funnels that  rush of water into narrow inlets that amplify the force of the tidal bulge. We’ll explain why the Med is different  in a sec, but first, this quick ad break. Thanks to our Presidents of Science, binorthedrunkdwarf, Charlie  Stanley, Harry Plumley, and Raz Tirosh for supporting this SciShow video!

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The Mediterranean Sea is… not one of those places  with spectacular high tides In the Mediterranean, there’s  sort of too much coastline. When you look at a map of the Mediterranean Sea, one of the first things you might  notice is how enclosed it is. While the Med does have a few points  where it meets larger bodies of water, those points are relatively narrow, which means that not a lot of water can  leave the Med or come in at any given time.

That means for tidal purposes, the Mediterranean is a bit like a bathtub. When the Sun, Moon, and  Earth start doing their thing and the tidal bulge moves over the Mediterranean, it can’t just borrow water from the  Atlantic to have a sea-wide high tide. The water that rises with the high tide has to come from somewhere  else in the Mediterranean.

And since the Moon is pulling on  the entire Mediterranean at once, this kind of results in the water  sloshing back and forth ineffectively, which is why tidal cycles in the Mediterranean are  shallower than in other parts of the world. And I really mean shallow! Most of the Mediterranean is considered  non-tidal for all practical purposes, with tides on average being about 30 cm high.

But while the tide in the Strait of Messina isn’t that high in the grand scheme of things, it’s known for having really  strong tidal currents, so strong that this area is actually  being considered for tidal energy. Those currents are constantly moving, rushing in both directions  through the Strait of Messina. The Strait’s geography contributes to those odd tides in the  Ionian and Tyrrhenian Seas.

When it’s high tide in the Tyrrhenian Sea, water flows into it from the Ionian Sea, through the narrow Strait of Messina. And vice versa - when the tide  in the Tyrrhenian Sea goes down, it rushes all the way back to the Ionian Sea, through the Strait of Messina. This happens on a semidiurnal period, which in tidal speak just means  that it happens about twice a day, creating a phenomenon that  looks a lot like a normal tide.

This “sea-saw” means that the tides in both seas are always going to be opposite each other. But that’s not all. The seas might be in about  the same part of the world, but they’re separated from each other by enough land to be chemically different.

The water in the Ionian Sea, for example, is saltier than the Tyrrhenian. Saltier water is denser than freshwater, and whenever waters of different densities mix, they can cause some really gnarly  whirlpools and other instabilities. These smaller disturbances, combined with the really  huge whirlpools in the Strait that come from all that  water rushing back and forth, are what make it a really  deadly place for mariners, making the Strait known for its wild,  strong currents, and deadly whirlpools.

It’s hardly surprising that the Ancient Greeks may have imagined monsters to explain the place. Makes you wonder what other sea  monsters might be lurking out there. [ OUTRO ]