Loricifera
Three species of Loricifera have been found in the sediments at the bottom of the L'Atalante basin in the Mediterranean Sea. The 'surface' of this basin is 3,500 meters down. They are the first multicellular organisms known to spend their entire lives in an oxygen-free environment. It is thought they are able to do this because they rely on organelles which, unlike normal mitochondria, do not use oxygen for their energy cycle. They are able to do this because they use hydrogenosomes to get their energy rather than mitochondria.