Every month, the Moon and the Sun align with the Earth during the full moon and new moon phases—this is called a syzygy. The combined gravitational pull of these two celestial bodies then causes the phenomenon of spring tides or "vives-eaux" tides on Earth. This is Newton’s law of universal gravitation at work.

These high tides are characterized by very large waves, strong currents, and a significant rise in water levels. It is a unique spectacle that has become a tourist attraction and even a business opportunity for some, a playground for the most adventurous, and a feared danger for others.

In Saint-Malo, the high tides bring enormous waves crashing against the pier and houses built facing the sea, while just a short distance away, the bay of Mont Saint-Michel fills with water, turning the rocky island into a world cut off for a few hours.