Mermaids are associated with the mythological Greek sirens as well as with sirenia, a biological order comprising dugongs and manatees. Some of the historical sightings by sailors may have been misunderstood encounters with these aquatic mammals. Christopher Columbus reported seeing mermaids while exploring the Caribbean, and sightings have been reported in the 20th and 21st centuries in Canada, Israel and Zimbabwe. The U.S. National Ocean Service stated in 2012 that no evidence of mermaids has ever been found.