Scientists have examined sediment cores from Antarctica, and have confirmed what was long hypothesized: Antarctica used to be much warmer and wetter, enough to support plant life. While the central region of the continent was likely lice bound the whole year, the coastal areas were able to support stunted plants, not unlike Greenland today.
Don't get too excited about the prospect of finding ancient civilizations though. Antarctica may have been able to support ancient peoples during this time, but that was 20 million years ago, long before mankind had evolved. Certainly, there were plants and animals living there at the time; we've found their fossils. It is possible that the fossils of all sorts of unknown lifeforms are buried in the Antarctic soil, but with three miles of ice on top, excavating them will be nigh impossible.
It is important to note that the climate of today is something of an anomaly, at least in geologic terms. Permanent polar ice caps were probably not present during much of Earth's history. Many creatures thrived in the polar regions of the world in previous eons. Dinosaur fossils have been found in Antarctica, although you have to keep in mind Antarctica was located considerably northward of where it is now. As we know now, Antarctica was somewhat habitable even in relatively recent terms.
Life has adapted to a warming and cooling Antarctica. It will do so again. Whether mankind is able to do so is another question.