In our planet’s history, we have witnessed five ice ages. The first one lasted 2.4-2.1 billion years ago, and evidence was found around Lake Huron. The second one, 850-630 million years ago, was “perhaps the most severe.” According to the website, “It is believed that during this period, the glacial ice sheets reached the equator, thus leading to a ‘Snowball Earth’ scenario.” The third, 460-420 million years ago, was traced through evidence in Andean and Sahara regions. The fourth, 360-260 million years ago, caused a long-term increase in oxygen levels and reduction in carbon dioxide, which led to global cooling, the exact opposite of what we witnessed over the past couple of centuries. The fifth started 2.5 million years ago, and is ongoing. There are several theories to explain the cycles and regularity of the ice ages, but there is little consensus between the scientists. In fact, many of the theories are inter-related and, hence, may together cause the expansion of the ice sheets. Milankovic cycles relate to cyclical variations of the Earth’s orbits. These lead to “changing distances from the Sun… and the changing tilt of the Earth’s axis,” which lead to a “redistribution of sunlight received by the Earth.” Hence, the planet’s temperatures change dramatically, with spectacular results. This theory gained credence because the timing of the glacial and interglacial phases in the past 400,000 years corresponds quite closely with the orbital variations.