The Moon creates and controls the tides.
Its gravitational pull on the oceans is so massive that it causes them to bulge upwards.
The Earth also gets pulled slightly towards the Moon, leaving another bulge of water on the opposite side.
The Earth spins through these bulges as it rotates on its axis.
It makes one full rotation every 24 hours, and so we get a high tide, and low tide, about every 12 hours.
The most extreme example of the Moon's effect on the tides occurs when the Moon is new, or full.
New Moon or full Moon
In line with Sun and Earth
Its gravitational pull combines with that of the Sun, giving much higher tides than normal.
These extreme tides are called spring tides.
Happening at the new and full Moon, spring tides occur roughly every two weeks.
The size of these spring tides varies with the annual cycle.
Most extreme at the Equinoxes
Least extreme at the Solstices
Where tides are funnelled inland – such as here at the Severn estuary – their effects become even more pronounced.
Severn Bore, United Kingdom
This is the infamous Severn bore – several times a year an exceptional high spring tide reverses the direction of the river, creating a moving wall of water 6 feet high.
On the other side of the Atlantic, a similar event occurs on the river Amazon – but on a much larger scale.
Amazon Bore, Brazil
Travelling hundreds of miles upriver, the Amazon's tidal bore scours the banks of its tributaries, leaving devastation in its wake.
But the world's most remarkable tidal bore takes place in China, on the river Qiantang, which reaches the sea just south of Shanghai.
Qiantang Bore, China
At the autumn equinox, an extremely high seasonal tide, assisted by seasonal winds, piles up the water into the mouth of the river.
This annual event attracts a daring crowd.
Known traditionally as the Black Dragon, it has claimed many lives, as warriors in boats have tried to ride the monster, and outrunning it is still a daredevil sport for the locals.
Qiantang's ferocious Black Dragon then returns to the sea.
And all this caused by the gravitational pull of the Moon, a quarter of a million miles away.