Leukonychia (or leuconychia), also known as white nails or milk spots, is a medical term for white discoloration appearing on nails. It is derived from the Greek words leuko (white) and onux (nail). The most common cause is injury to the base of the nail (the matrix) where the nail is formed.
Extinction Events. 1  There were many small-scale, sporadic extinction events during the Cambrian Period. 2  A large-scale mass extinction event occurred approximately 500 million years ago. 3  Scientists believe was caused by a significant decrease in global temperature. A large-scale mass extinction event occurred approximately 500 million years ago. 2  Scientists believe was caused by a significant decrease in global temperature. 3  Decrease in global temperature created an ice age-further eliminated many of the oceanic species.
Artist's impression of a major asteroid impact. An asteroid with an impact strength of a billion atomic bombs may have caused the extinction of the dinosaurs. A global catastrophic risk is a hypothetical future event that has the potential to damage human well-being on a global scale. Some events could cripple or destroy modern civilization. Any event that could cause human extinction is also known as an existential risk.