Supposedly there's evidence that at one time the entire surface of the earth was covered in a kilometer thick ice sheet (super ice age about 750 million years ago, see web page: http://millennium-debate.org/ind10ap035.htm). As far as I understand, besides temperature (at least partly a function of what the sun is up to) the only other variable affecting sea levels would be the amount of volcanic and tectonic upthrust (and decline) which the seafloor and land masses have undergone.