This instalment details coastal environments and the effect of tides, of which the highest can be found in the Bay of Fundy in North America. In places, erosion is causing the land to retreat, while in others - such as the tropics - the expansion of mangroves causes it to advance. Mussels keep their shells closed at low tide to deter attackers but the oystercatcher is adept at dealing with them. Other estuary wading birds, which have developed a multitude of techniques for gathering food from mud flats, include godwits, curlews, dunlins, Ringed Plovers and avocets. While glasswort grows on many European tidal banks, the mangroves of the tropics are extensive. The largest forest is in the Sundarbans at the mouth of the Ganges and is 370 square metres in size. Where waves meet rocks and cliffs, the bands between low and high tides are narrow, and creatures have developed according to their dietary and safety needs. Mussels are preyed on by starfish, and so ensure that they are out of reach at low tide. Barnacles are higher still and feed on microscopic particles. On a Costa Rican beach, Attenborough observes female Ridley turtles arriving at the rate of some 5,000 an hour to deposit their eggs. Finally, he discovers the largest turtle, the giant leatherback, also laying eggs. He remarks that despite its great size, little is known about it - except that its eggs are easily plundered, thus making it an endangered species.