Amphicoelias (meaning 'doubly hollow', from the Greek amphi: "on both sides", and koilos: "hollow, concave") is an extinct genus of herbivorous sauropod dinosaur that includes what may be the largest dinosaur ever discovered, A. fragillimus. Based on surviving descriptions of a single fossil bone, A. fragillimus may have been the longest known vertebrate at 40–60 meters (131–196 ft) in length, and may have had a mass of up to 122 metric tons (135 short tons), rivaling the heaviest animal known, the Blue Whale. However, because the only fossil remains were lost at some point after being studied and described in the 1870s, evidence survives only in drawings and field notes.