Since the 1980s, many biologists have concluded that the earth is in the midst of a massive biodiversity extinction crisis caused by human activities. Yet fewer than 1,000 of the planet's 1.9 million known species are officially recorded as extinct. Skeptics have therefore asked "Is there really a crisis?" Mammals and birds provide the most robust data, because the status of almost all has been assessed. Invertebrates constitute over 99% of species diversity, but the status of only a tiny fraction has been assessed, thereby dramatically underestimating overall levels of extinction. Using data on terrestrial invertebrates, this study estimates that we may already have lost 7% of the species on Earth and that the biodiversity crisis is real.