One reason oceanographers analyze the sediment on the ocean floor is to see how long-term changes in Earth's temperature have affected the depth of the ocean. By analyzing the remains of sea animals in old layers of ocean sediment, oceanographers can determine the depth of the ocean in the past. They've analyzed hundreds of such layers, including some from the coldest periods of Earth's history - the ice ages. What they've found is that during the ice ages, the amount of water in the oceans decreased. Water levels in the ocean dropped by about 400 feet. Water from the ocean evaporated and became frozen in continental glaciers, so it didn't drain back into the ocean. When temperatures eventually rose again, the glaciers melted, and the oceans returned to their former depths. Analysis of sedimentary data indicates that periods of glacial freezing and melting occurred in regular cycles of 20,000, 40,000 and 100,000 years. Oceanographers are interested in the history of seawater levels because they hope to use this historical data in order to predict the possible effect that global warming could have on seawater levels. If industrial pollutants are capable of heating global temperatures to the point that glaciers begin to melt, it is urgent for us to know precisely how high sea levels will rise as a result.