The path which led to the discovery of heavy hydrogen and heavy water was every bit as tortuous. In 1913 Arthur Lamb and Richard Lee at New York University were trying to improve measurements of the density of water; this was a very important quantity to know accurately because of its importance as a standard. They were attempting an accuracy of 200 ppb, but were not able to get agreement between samples taken from different geographical locations to better than 800ppb. They did not grasp the significance of this discrepancy - the abundance of deuterium varies due to rates of evaporation and condensation - despite the fact that the concept of isotopes as developed by Frederick Soddy and Kasimir Fajans was a significant piece of scientific news that year.
In 1929 William Giauque and Herrick Johnston discovered O and O. Two years later Harold Urey at Columbia constructed chart of missing and known isotopes and from gaps in the pattern mused on the possible existence of 2H, 3H and 5He