Methane hydrates are mounds of ice (crystallized structures of methane & water) that can form under conditions of low temperature and high pressure.  this methane hydrate mound (right side of photo), discovered by Charles Fisher 700 meters deep in the Gulf of Mexico, is over 6 feet in diameter and has thousands of polychaete worms living on its exposed surface.  The structures on the left side of the photo are tube worms.  (Photo credit: Charles Fisher, Penn State)