PSYCHOLOGY, sociology, or biology? Why is it that we are drawn so strongly to the sea, to the ceaseless ebb and flow? Talk to psychologists, and they might tell you about the womb; talk to sociologists, and they will say that it is all about the culture of re-creation: after all, nobody in this country bothered with the seaside until the later 18th century, when it became fashionable in high society.

Meanwhile, the biologist might provide a hypothesis based on an assumption that our species developed on an island, and that we retain an affinity with the sea. How else could one explain our ability to swim under water - something our nearest mammalian relatives cannot do?

All these theories, and more, were picked over by Mike Williams in The Why Factor (BBC World Service, Friday), one of those unpretentious mini-documentaries that the World Service churns out week after week. It is unclear, however, what genetic mutation produced Hanli Prinsloo, the South African "free-diver" who is capable of holding her breath for six minutes, and diving to 65 metres.

We heard about the complex physiological responses to this kind of adventure. What happens to the heart, lungs, and brain is, frankly, alarming. In contrast, most people's experience of the sea is a restful one - indeed, it can be shown how the sound of waves has a calming effect on the heart rate.

Yet the sea also represents chaos and destruction; at least, it did to the writer W. G. Sebald, whose travelogue The Rings of Saturn presents an analogy between the effect of the sea on the Suffolk coastline, and the endless history of human enmity and war. The best thing about Between the Ears: Sebald's apocalyptic vision (Radio 3, Saturday) were the extracts from Sebald's book, written in 1995 in a prose unlike any other - the earnestness of a Victorian traveller, suffused with a contemporary sense of malaise.

Less successful was the doc-umentary element of this programme, which told of the laudable ambition of a German theatrical troupe to learn more of Sebald's work by retracing his steps. The trouble with this project is that Sebald's journey is through a psycho-geography rather than a real environment. You can visit the places Sebald visited, but you cannot, by doing so, recreate that particular atmosphere of melan-cholia that he projects on to it. We learned little from the actors' experience, except that this is a modern literary classic that de-serves our profoundest respect.

An urban psycho-geography was the basis for Craig Taylor's drama-documentary Ashes, the first in a new late-night series, Dreaming the City (Radio 4, Wednesday of last week). Embedded into the real environment of London, this fictional narrative told of two people brought together by their duty to spread a friend's ashes across the city. We heard from various "real" people, and we began to speculate about why these two characters had been chosen for this task.

Except for some irritating musical underscoring, this was a beautiful piece of work, genuinely atmospheric and innovative. I was listening online in the full glare of a sunny day; in its 11.15 p.m. slot, this would have been a perfect fit.