In the beginning...the Earth had essentially no free oxygen in its atmosphere. The Great Oxygenation Event came around 2000 million years ago, when cyanobacteria produced enough oxygen for it to accumulate, but only in the past 800 million years did an explosion of multicellular animal life take place. The intervening period of not much happening is often called, rather approximately, "the boring billion". Now, Noah Planavsky of Yale University and colleagues have shown that it coincides with a low atmospheric oxygen content, improving previous upper limits by at least an order of magnitude. Looking at the oxidation of chromium isotopes in ancient sediments, they found that oxygen levels were only 0.1% of those today – while associated with the boring billion, this is not boring at all.