Anaerobic - without oxygen.
Oxygen is present almost everywhere on Earth, in the air and dissolved in water. It allows us to live - and also to decay.
Oxygen is required for many of the natural processes that we take for granted, including the breakdown of organic material and the rusting of iron, so in an atmosphere of very low oxygen (for example, in silt, peat bogs and outer space) these processes are slowed or even stopped.
Hence we find ancient remains preserved far beyond their expected years in these environments.
An interesting and important point was made by CapnTrippy: At one time the entire atmosphere was anaerobic I think...The dominance of oxygen produce species caused one of the greatest die-off ever...Maybe someone would like to enlarge on this?