During cycles of global cooling , such as the most recent ice age , enough sea water became concentrated in the ice caps of the Arctic and Antarctica that the subsequent drop in eustatic sea levels exposed shallow sea floors that have subsequently re-flooded . During cycles of global cooling , such as the most recent ice age , enough sea water became concentrated in the ice caps of the Arctic and Antarctic that the subsequent drop in eustatic sea levels exposed shallow sea-floors that have subsequently re-flooded .