About 80 million years ago, no land-based ice sheets existed. Also, a larger proportion of the world's ocean crust rode higher than now on underlying mantle, so oceans were shallower.
Computer models suggest that sea level then was about 170 meters higher than today, says R. Dietmar Müller, a geophysicist at the University of Sydney. Many areas that are now dry, including northern Europe, were covered with shallow seas (dark blue areas, top map). If, overall, ocean basins continue to drop as expected, 80 million years from now sea levels will have dropped another 120 m, exposing vast swaths of continental shelf (dark green areas, bottom map), Müller and his colleagues report in the March 7 Science. If today's ice sheets melt, sea level will drop only 70 m.