US and Australian scientists think they know why glaciers existed at the Earth's equator 600 million years ago and why the Earth's polar regions were warmer then than they are today. The researchers at Pennsylvania State University and the University of Adelaide say that the reason was that the Earth used to have a pronounced tilt up to 55 degrees, compared to 23.5 degrees today. They believe that the expansion and contraction of giant ice sheets around the equator might have caused a positive feedback loop to the position of the Earth's gravitational tilt (Nature 396 453).