The International Union for Conservation of Nature (IUCN) is the premier global biodiversity conservation organization. Its Red List is a rigorous vehicle for assessing the conservation status of plant and animal species. However, although all animal and bird species recognized by IUCN have been evaluated, only a tiny fraction of invertebrates have been evaluated. As a measure of the numbers of extinct species (since around the year 1500) the Red List is probably quite accurate for birds and mammals, but severely underestimates the numbers for invertebrates. Nonetheless, molluscs stand out as the major group most severely impacted by extinction, with 297 of the 744 animal species listed as extinct in the third issue of the 2016 Red List. Here we review efforts to obtain a more realistic, albeit less rigorous, assessment of the numbers of extinct mollusk species. Our approach has been based on bibliographic research and consultation with experts, rather than following the highly detailed but restrictive IUCN Categories and Criteria. In 2009, this led to an assessment that 533 mollusk species were extinct, far more than the number on the Red List. In the present study we revisited this approach and here list 638 species as extinct, 380 as possibly extinct, and 14 as extinct in the wild, a total of 1,032 species in these combined categories, and more than twice as many as listed by IUCN in these categories. However, this approach only considers species for which information is available; it is therefore biased. In a study published in 2015 we developed an alternative approach, based on a random global sample of land snails, and estimated that 3,000-5,100 mollusk species have gone extinct. We review the main reasons for these extinctions: habitat destruction, impacts of introduced species, exploitation and collecting, and, potentially, climate change, and discuss relevant case studies. Oceanic island land snails, especially those of Pacific islands, have suffered the greatest proportion of the extinctions, with some species having gone extinct before being discovered and described scientifically. The Amastridae, an endemic Hawaiian family of 325 recognized species, may have lost all but 18 species. We outline the phases in this catastrophe: 1) pre-human and/or prehistoric extinction, either natural or anthropogenic, with species known only as fossils/subfossils; 2) extinction due to habitat destruction and introduction of a number of alien species by Pacific island people as they settled the islands; 3) extinction due to extensive habitat destruction and introduction of highly destructive invasive alien species following colonization by Westerners; 4) extinction following the advent of large-scale agriculture at the end of the 19th Century, at the time of a major increase in the land snail extinction rate globally; 5) extinction due to increased military activity, tourism, commerce, urbanization and the concomittant rapidly increasing introduction of invasive species after the Second World War. Extrapolating from our assessments of mollusks, we estimate that approximately 7.5-13% of all species have gone extinct since around year 1500. This is orders of magnitude greater than the 860 (0.04% of 2 million) listed as extinct by IUCN (2016). The biodiversity crisis is real.