Hat goes up can also come down, particularly when one discusses changing sea levels. Researchers from Southampton University describe their efforts to determine sea levels during past glacial maximums in the current issue of Nature. Continental ice-sheets draw vast quantities of water out of the oceans, causing sea levels to plummet. Levels during the last glacial maximum 20,000 years ago were around 120 metres lower than their present levels. Elaborate methods are used such as looking at oxygen isotopes in the shells of sea floor organisms but these give indications only up to 140,000 years ago. Southampton's Dr Eelco J Rohling and colleagues push the records back 500,000 years through their combination of evidence of very high salinity conditions in the glacial Red Sea with models of water flow through the Strait of Bab-el-Mandab which links the Sea to the open ocean.