On the big tides the ocean crowds into a shallowing river mouth until it rears up and breaks, then runs miles inland as a moving wall of brown water — and in each of these five places the whole town knows the hour it is due.
The Qiantang bore is the largest on Earth: up to nine metres high, racing inland at 40 km/h. On the eighteenth day of the eighth lunar month, tide-watchers pack the fish-scale stone seawall at Yanguan — Ming-built, Qing-rebuilt — in their tens of thousands.
The Petitcodiac bore once ran a metre or more high until a 1968 causeway choked the river with silt and cut the wave to a ripple. The gates were opened in 2010 and a bridge replaced the causeway in 2021 — the wave, and the river's fish, came back.
The Severn bore forms around 260 times a year and was measured at 2.8 metres in 1966. It usually spends itself on the pair of weirs either side of Alney Island at the city's edge, though the biggest tides carry it further up; in 2006 Steve King surfed one for 12 km.
This bore is a freshwater oddity: the Atlantic tide travels far up the Rio Guamá and throws up a wave right where the Rio Capim joins it, with the sea nowhere near — 130 km and two hours of road from Belém. The town has hosted a national pororoca surfing championship on that wave since 1999.
The Bono runs four to six metres high on the big tides, and villagers call the train of waves the Seven Ghosts — spirits blamed for smashing boats, until, the story goes, Dutch gunners shelled one of the seven to pieces; six have run ever since.
Think you'd have found them? Play today's expedition, or replay this one from the archive.