Scientists in Japan have identified a new taste on the human palate, though it sounds even more quizzical than umami. The Smithsonian is (very) loosely translating kokumi as "heartiness" or "mouthfulness," though further reading describes kokumi not as its own flavor, but rather as a flavor enhancer when combined with the right ingredients. Some of these ingredients include compounds like "calcium, protamine (found in milt, or fish sperm, which is eaten in Japan and Russia), L-histidine (an amino acid) and glutathione (found in yeast extract)."
Scientists are still working to understand how the compounds work, though they believe calcium channels on the tongue (just discovered in 2008 as a way the body regulates calcium intake) play a large role. Much of the research is now focusing on the presence of calcium in certain foods.