ABSTRACT
At the top of one of the highest hills in the Finnish town of Turku, overlooking the medieval cathedral, is a plaque marking the level of the sea in 5000 BC. Successive sea-levels from that date to recent times are marked with more plaques as one descends the steep, rocky slope to the river, some 50 metres below. A thousand miles to the south-west, one can look out over the level, fertile polderland of North Holland from the approach road to the massive barrier dam which runs arrow-straight for 30 kilometres across the waters to the Frisian mainland, and be startled to realise that all the land within sight is several metres below the level of the waters of the Zuider Zee which lap against the dike.
