A. The Bay of Fundy lies on Canada's Atlantic coast, separating the province of New Brunswick from Nova Scotia. It is home to the highest tides on Earth. Twice a day, the water at the head of the bay rises and falls by as much as sixteen metres, roughly the height of a five storey building. During each tidal cycle, about 160 billion tonnes of seawater move in and out of the bay, more water than the combined flow of all the world's freshwater rivers over the same period. At low tide, boats in some harbours settle onto bare mud, and wharves tower above the decks they were level with a few hours earlier.

B. The bay owes these extremes to an unusual coincidence of shape and timing. Its length and depth give the water inside a natural rocking rhythm, a slow slosh from one end to the other that takes about thirteen hours, close to the interval between one high tide and the next. Each arriving ocean tide therefore pushes water that is already swinging the same way, just as a playground swing climbs higher when every push matches its rhythm. The bay also narrows and grows shallower toward its head, squeezing the incoming water into a smaller space and lifting it higher still.

C. The endless rise and fall has reshaped the land itself. At Hopewell Rocks in New Brunswick, the tides have carved the shoreline into sandstone pillars, wide at the crown and pinched at the base, with trees still growing on their tops. Visitors can spend the morning walking on the ocean floor among the pillars, then return in the afternoon to watch the same ground vanish under more than ten metres of water. The advancing tide is strong enough to push a wave upstream into several rivers that empty into the bay, briefly forcing their currents to run backwards, and every few years erosion brings one of the famous pillars crashing down.

D. The bay's rhythm also supports a remarkable web of life. The retreating water uncovers vast mudflats where tiny mud shrimp burrow near the surface, thousands of them in a single square metre. Every summer, hundreds of thousands of semipalmated sandpipers pause on these flats during their migration, feeding almost around the clock until they nearly double their weight, fuel for a nonstop flight of several days to South America. Whales gather in the outer bay to feed where the currents stir the water, and engineers have tested underwater turbines in its narrow passages, hoping to turn the planet's greatest tides into electricity.