Banded iron formations (BIFs) became very abundant early this period. BIFs were formed as anaerobic algae produced waste oxygen that combined with iron, forming magnetite (FeO, an iron oxide). This process cleared iron from the oceans, presumably turning greenish seas clear. Eventually, without an oxygen sink in the oceans, the process created the oxygen-rich atmosphere of today. This event is known as the oxygen catastrophe, which according to some geologists triggered the Huronian glaciation. The Huronian glaciation took place in the Siderian between 2400 Mya and 2300 Mya.