 I'm on the plage Bonaparte in France. It's about six o'clock and the tide's coming in. This is the biggest tide of the year. A high tide of water will be 12 metres higher than it was a low tide. The Bay of Saint-Pierre has one of the biggest tides in the world. So why does this place in Britain play host to such... massive tides? The first question is why we get tides at all. Imagine this circle is the planet Earth, and there aren't any continents. The whole planet is covered in ocean. This layer of water can get pulled around, for example by the gravity of the moon, and this creates two so-called tidal bulges, one on the same side of the Earth as the moon and one on the opposite side. However, not all tides are created equal. The other large source of gravity acting on the Earth's oceans is the sun. In order to get the biggest tides, you want the sun and the moon to be acting together, so they need to be aligned, either on the same side of the Earth, so at a new moon, or on opposite sides of the Earth, at a full moon, like the one we had last night. This means you get bigger tides at certain times of the month, but even the combined gravity of the sun and the moon is only enough to give you a tide of two or three metres. This black mark shows you the approximate height of high tide on this beach, and as you can see, we're a lot more than two metres above the sand, and the sea is way out there. This is because, actually, we do have continents, and the shape of those continents, and the seabed, can make for tides that are much higher or lower than the simple picture from earlier would suggest. The reason the tides here are so enormous is because of the shape of the coastline, which creates a phenomenon called a tidal resonance. Resonance occurs when you match a frequency to the fundamental frequency of an object. The fundamental frequency is the rate at which something likes to wobble, so if you wobble it at exactly that frequency, then the vibration will tend to amplify. For example, an opera singer can make a wine glass vibrate by matching the frequency of her voice to the pitch of the fundamental frequency of the wine glass. If she sings the wrong note, then not very much happens. But if she can get the frequency just right, then the wine glass wobbles really hard, as you can see from the straw bouncing around inside it. What's this got to do with the tide? Well, you can imagine the two tidal bulges on opposite sides of the earth, acting as a giant tidal wave, which goes up and down once every 12 and a half hours. If you get a bay that's just the right size and shape, then that wave can amplify, like the vibrations in the wine glass, moving the water further and faster, and turning a meager 2 metre tide into a 12 metre monster like this one. Understanding the tides has been vital to sailors for centuries, and now we can use powerful computer models to predict where to place underwater turbines to use some of this vast power to generate renewable electricity. Taking a look out at the tides is also important for your personal safety. Can I get out now?