Mare's Eggs (Nostoc pruniforme) is a form of blue-green algae. It is actually a colony of small cells living in a gelatinous sphere that ranges in size from 2 to 6 inches in diameter. It ranges in color from near black to a greenish-gray and is pretty cool looking. Mare's Eggs are found all over the world, usually in very cold water. One place that these structures can be found is at Mare's Egg Spring in Klamath County in Southern Oregon. The spring is very cold (41 degrees F) and clear and the bottom is covered with Mare's Eggs. Mare's Eggs are considered endangered in Klamath County, and are not harvested as a food supplement like they once were.
Mare's Eggs are one of the Cyanobacteria or blue-green algae. Blue-green algae are known for their ability to fix (harvest and deposit) Nitrogen from air and water, and for the ability to survive in extreme temperatures, both hot and cold. Blue-green algae are often the only organisms surviving in geysers and hot springs. The brilliant colors found in Yellowstone hot pools and at Mammoth Hot Springs are due largely to the presence of Cyanobacteria.
My own experience with a Mare's Egg colony is rather unpleasant. One day while getting my lunch out of the refrigerator at work, I noticed a plastic container labeled "Mare's Egg". Thinking it was an actual reproductive structure from a female horse, I looked inside. I saw a gelatinous mass, and closed the container, feeling sure that I'd seen an ovum from a mare. Well, Mare's Egg algae colonies are well known for lasting a long time in cold water. That container sat in the refrigerator for months. I was sure that what I thought was something from a horse was getting disgusting and smelly. It eventually got so bad, in my mind at least, that I couldn't use the refrigerator anymore without losing my appetite. Finally I asked one of the people who I suspected of placing the horse ovum if they would kindly remove it so I wouldn't be so grossed out all the time. He laughed and told me what this "Mare's Egg" really was. And sure enough, when I looked, the cluster was still firm and non-odorous.