Antarctica was once a swampy forest with an average temperature of about 12C, scientists have concluded.
The study, published in the journal 'Nature', found that the continent had a much more temperate climate 90 million years ago - similar to that of parts of New Zealand today.
Scientists discovered the previous temperature of the Antarctic by looking at soil samples from the mid-Cretaceous period (80-115 million years ago).
The team said that an analysis of preserved roots and other plant remains in the soil suggests that the world at that time was warmer than previously thought.
They analysed an ice core extracted from the seabed near the Pine Island and Thwaites glaciers in west Antarctica.
The team found well-preserved forest soil, including plant pollen, spores, a dense network of roots and the remains of flowering plants.
It is not yet know what caused the climate change and ice sheets to form.
Professor Tina van de Flierdt, an Earth sciences expert from Imperial College London and one of the study's authors, said: "The preservation of this 90-million-year-old forest is exceptional, but even more surprising is the world it reveals.
"Even during months of darkness, swampy temperate rainforests grew close to the South Pole, revealing a warmer climate than expected."
Professor Ulrich Salzmann, a paleoecologist at Northumbria University, said: "The numerous plant remains indicate that 93 to 83 million years ago, the coast of west Antarctica was a swampy landscape in which temperate rainforests grew - similar to the forests on New Zealand's South Island."
The findings also indicate that carbon dioxide levels were higher than had been expected.