Photo credit: Keith Slausen/US Forest Service
According to some estimates, a species is lost every 20 minutes. This shocking number considers factors like habitat loss but also includes approximations of the planet's total number of species, known and unknown.
Actually declaring a species extinct, however, is a long and laborious process that requires evidence beyond a reasonable doubt the plant or animal has disappeared from the face of the earth. The difficulty of this process is highlighted by the fact that only 875 extinctions have been officially documented since 1500.
And a few of those 875 have made miraculous reappearances.