Last week, a group known as Whale and Dolphin Conservation (WDC) received the above video from YouTube user Peter Hofmann. The video shows a pure white harbor porpoise -- a creature that has only seen sighted in the wild 15 times in the past 100 years, according to WDC. Hofmann spotted it in the Baltic Sea, close to Denmark's Great Belt, one of three major straits that run through the island-rich country.

"Harbor porpoises are shy, elusive sea mammals," says National Geographic, so the fact that one was actually spotted -- and caught on film -- is truly remarkable. This one even seems to enjoy its moment in the spotlight, as it swims along for a bit quite close to the boat.

Normally gray in color with white bellies, harbor porpoises can dive over 655 feet (about 200 meters) but tend to stay in more shallow water (thus their name). They are also some of the smallest cetaceans -- the order of mammals that includes dolphins, whales and porpoises -- and grow to be about 5 feet (1.5 meters) long, with a weight of 121 pounds (55 kilograms) on average, according to National Geographic. The magazine also says the mammals surface about every 25 seconds "to breathe with a distinctive puffing noise that resembles a sneeze."

Porpoises differ from dolphins in their appearance, with shorter "beaks" and smaller mouths. According to the National Ocean Service, there are 32 dolphin species and only six porpoise species.

"White harbor porpoises are very rare, so every sighting is a major event," Fabian Ritter, a marine biologist at WDC, said on the group's website. "It is important to report such sightings. They are of great scientific value to us."

(Via io9)