Throughout its history, the Earth has cycled between cold glacial periods (during which large masses of ice, known as glaciers, cover much of the world's land area) and warm interglacial periods (which lack widespread glaciers). Humans have lived through many such cycles, which take place over thousands of years. While glacial periods made survival more difficult, they also lowered the sea level, easing human colonization of the world. We currently live in an interglacial period, which began ca. 10,000 BC.