This abstract from a 2001 research paper in a neurobiology journal sums up the current thinking...
`Orientation, navigation, and homing are critical traits expressed by organisms ranging from bacteria through higher vertebrates. Sensory systems that aid such behavior have provided key selective advantages to these groups over the past 4 billion years, and are highly evolved; magnetoreception is no exception. Across many species and groups of organisms, compelling evidence exists that the physical basis of this response is tiny crystals of single-domain magnetite (Fe3O4). It is the opinion of the authors that all magnetic field sensitivity in living organisms, including elasmobranch fishes, is the result of a highly evolved, finely-tuned sensory system based on single-domain, ferromagnetic crystals.' LINK
On a different topic..
`Ozone layer: Concern grows over threat from replacement chemicals' LINK
`Substances used for air conditioning in almost all new cars are building up in the environment and may pose a threat to human health, researchers say. These "ozone friendly" chemicals have been introduced to replace products that were damaging the ozone layer. Now widely used across industry, these alternatives do not break down in the environment. Scientists have now found increasing levels of these chemicals in Arctic ice samples dating back to the 1990s..'.
`....Canadian researchers, studying ice samples from the Arctic dating back to the 1990s, have found "dramatically" increasing levels of ozone replacements called short chain perfluoroalkyl carboxylic acids (scPFCAs). "We're seeing much, much larger levels, on the order of 10 times higher now than we saw before the Montreal Protocol," said Prof Cora Young, from York University in Toronto, the study's corresponding author. "We don't know a lot about them and their potential toxicity, but we do know that we are committing the environment to a great deal of contamination."
