Global biodiversity, which is the total number of plant and animal species existing on the planet at any one time, can only be estimated; and only very roughly estimated at that. Undoubtedly, many species remain to be discovered. Some of these, such as insects and microscopic life, are small enough to have escaped our notice, while others dwell in areas we have only begun to explore; the species inhabiting the deepest ocean depths, for example. It must also be remembered that we are in the midst of a mass extinction event. Species are now disappearing at a rate estimated to be between 1,000 and 10,000 times faster than the average for the history of life on earth. Taken together, these two uncertain elements prevent the global biodiversity estimate from becoming much more than an educated guess. The current best estimate is somewhere in the neighborhood of 100 to 200 million species.