Spring tides occur at full moon and new moon, when the sun and moon align to combine their gravitational pull, significantly increasing the energy of tides and associated phenomena such as bores, rapids, waves and whirlpools. William Thomson visits the world’s most dramatic tidal landscapes, explaining by means of stylish graphics, environmental science and entertaining anecdote the twice-daily motion of coastal waters ranging from the Skookumchuck Narrows in British Columbia to the giant waves over the Nazaré Canyon in Portugal.