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When did Antarctic ice sheets begin to form?

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Messinian Stage — Tertiary Period Gypsum cones, which resulted from the evaporation of the Mediterranean Sea during the Messinian Salinity Crisis, in the Sorbas basin, Spain.

The process of continental-scale glaciation in Antarctica was initiated by a rapid global cooling trend that occurred across the Eocene–Oligocene boundary, approximately 33.9 million years ago.12

Following this onset, ice sheets developed at sea level on West Antarctica during the early Oligocene. While these ice sheets persisted for a time, they were subject to climatic fluctuations, including a rapid warming event 27 million years ago. The Antarctic Ice Sheet subsequently expanded to cover most of the continent by the middle of the Miocene Epoch, about 13 million years ago, eventually reaching a state of virtually complete glaciation by the late Miocene, approximately 5.5 million years ago.12