How many species might still be undiscovered?

Around two million species have been identified and classified so far, most of them mammals, birds and insects with about 15,000 new species found each year. Current estimates for the total number waiting to be found range from five to 100 million. Most of these will be fungi and bacteria. There are also lots of insects that are virtually identical to look at but differ enough genetically to count as different species. This means that classifying them is a very slow process and it's almost impossible to know when you're finished. New mammal species are found much less frequently. One recent one was a kind of tree rat found in Peru in 2000.