This timeline covers the Holocene extinction event. Holocene is the epoch which began approximately 11,550 calendar years before present (about 9600 BC) and continues to the present. And the Holocene Extinction Event is a name customarily given to the widespread, ongoing mass extinction of species during the modern Holocene epoch. The large number of extinctions span numerous families of plants and animals including mammals, birds, amphibians, reptiles and arthropods; a sizeable fraction of these extinctions are occurring in the rainforests. Since most extinctions are likely to go undocumented, scientists estimate that during the last century, between 20,000 and two million species have become extinct, but the precise total cannot be determined more accurately within the limits of present knowledge. Up to 140,000 species per year (based on Species-area theory) may be the present rate of extinction based upon upper bound estimating.