Twice a day, the sea goes up, and then goes down in the Mont St Michel Bay.  It is the tide. The Moon and the Sun attract all liquid on Earth.  It is the law of Mother Nature. As the Moon is much more close to Earth than the Sun, its action is the strongest, in spite of its small size.

The level of water increases or decreases according to the influence of the Moon. Each time the Moon is physically close to a part of the ocean or a sea, the area is attracted by it, inflates and discovers the coasts. Meanwhile, the terrestrial attraction retains water on the Earth.

There are 4 major types of tides:

- Regular semi-diurnal tides on the coasts of Europe and in the Atlantic

- Semi-diurnal tides with diurnal inequality in the Indian Ocean

- Mixed tides in the Pacific

- Diurnal tides in the Gulf of Mexico

 In order to understand what happens in the Mont St Michel Bay, you have to take in consideration that a lunar cycle is 24h50 min that creates two high tides, of appreciably equal heights and two low tides appreciably equal heights too.  The high tide is created when the Moon passes to the top of the English Channel, it attracts water towards it, as a magnet and the sea level rises along the coasts: it is the high tide.  By a curious phenomenon, the high tide takes place also on the other side of the Earth.

 Six hours later, the Earth made a rotation of a quarter of turn on itself but the Moon practically remained at the same place in space.  The coast is not any more under the Moon.  The level of water is gone down again.  It is the low tide.

Because the cycle of the moon is 24 hours and 50 minutes, you can expect to have the same event 50 minutes later the following day (if you. the low tide at 10.50am on Monday, you will expect to have a low tide around 11h40am the following day.

The spring and fall tides are semi-diurnal tides. The maximum amplitudes (deepness between the low tide and the high tide) on earth are observed and are gigantic as you can expect 44 feet deep between the low tide and the high tide in the Mont St Michel Bay and the water. During those types of tide, the water leaves the coast for up to 10 miles. And when the water comes back, because of the inclination on the bay bed, the speed can rise up to a horse galloping.

 The tide is not an exact science it varies with the weather but there are a series of rules to follow in order to be safe. You will find in any local bar/Deli, a timetable of the important tides that may affect your stay. It is also good to know the tides if you want to go fishing. Don’t expect to go fishing in low tide when you boat is stock in the sand without water in miles around for at least six hours. 

Last good advice, before you park your car close to the sea (like on the parking lots at the foot of the Mont St Michel, be aware of the sign indicating when the tide is coming back in order to avoid the bad surprise to only see the antenna of the radio of your car pointed out of the water during full tide: the insurance company will not reimburse you: that s the law!