Ice worms are annelids of the genus Mesenchytraeus. They are the only annelid worms known to spend their entire lives in glacial ice. They include M. solifugus, M. harrimani, M. kuril, M. maculatus and M. obscurus.
The first ice worms species were discovered in North America in 1887 in Alaska, on the Muir Glacier. These glacier ice worms can be found on glaciers in Alaska, Washington, Oregon and British Columbia. They have not been found in other glaciated regions of the world. The specific name solifugus is Latin for "sun-avoiding," as ice worms retreat underneath the ice before dawn. Enzymes in ice worms have very low optimal temperatures, and can be denatured at even a few degrees above 0 °C (32 °F). When ice worms are exposed to temperatures as modest as 5 °C (41 °F), their membrane structures disassociate and fall apart (i.e., "melt") causing the worm itself to "liquefy." Ice worms are several centimeters long, and can be black, blue, or white. They come to the surface of the glaciers in the evening and morning. On Suiattle Glacier in the North Cascades, population counts indicated over 7 billion ice worms.
Ice worms were described earlier by Evliyâ Çelebi in Turkey. "In 1648 Evliyâ observed these worms on Syna's Jebel-I Hayy' u's-selc, the 'Mountain of Hard Snow', and tells us that the snow gatherers presented them as gifts to physicians. Although the phenomenon had been described by many geographers and travelers since antiquity, it was dismissed by scientists as a myth until late nineteenth-century research proved that a species of dark brown or black worm (Mesenchytraeus solifugus) does indeed live in glaciers and permanent snow." 
It is not known how ice worms tunnel through the ice. Some scientists believe they travel through microscopic fissures in ice sheets, while others believe they secrete some chemical which can melt ice by lowering its freezing point, like an antifreeze. They feed on snow algae.
In popular culture
- Scottish-born Canadian poet of the Yukon Robert W. Service wrote the poem "The Ballad of the Ice-worm Cocktail", in which a fake ice worm made of spaghetti is the subject of a bar bet. This may have contributed to the impression that ice worms are mythical creatures.
- An old Canadian song "When the Ice Worms Nest Again" jokes about how cold it is in northern Canada. (Titles vary, using "iceworms," "ice worms" or "ice worm.") Another song of the same name is by the Canadian group Jenny Omnichord; it is highly factually accurate.
- Organisms similar to ice worms have appeared in science fiction in the short story "Glacial" by Alastair Reynolds and the novel Fallen Dragon by Peter F. Hamilton.
- From the Star Trek: Enterprise TV series, "Ice-bores" were worms that lived in the ice-packs of Andoria.
- In the annual February Ice Worm Festival of Cordova, Alaska, a long imitation ice worm is paraded through the streets like a Chinese new year dragon dance.
- Kathryn A. Coates; Jan M. Locke; Brenda M. Healy; Mark J. Wetzel (August 26, 2008). "The aphanoneuran and clitellate Annelida occurring in the United States and Canada: families Enchytraeidae and Propappidae". University of Illinois at Urbana-Champaign. Retrieved January 22, 2010.
- Shain, Daniel H.; Carter, Melissa R.; Murray, Kurt P.; Maleski, Karen A.; Smith, Nancy R.; McBride, Taresha R.; Michalewicz, Lisa A.; Saidel, William M. (2000). "Morphologic characterization of the ice wormMesenchytraeus solifugus". Journal of Morphology 246 (3): 192–7. doi:10.1002/1097-4687(200012)246:3<192::AID-JMOR3>3.0.CO;2-B. PMID 11077431.
- Evliyâ, Çelebi (1996–2007), vol. 9, p. 281
- Service, Robert William (1910). The Trail of '98. New York: Grosset & Dunlap. p. 209.