The gaseous diffusion process remained more encouraging, although it too had technical obstacles to overcome. By the end of 1943, Urey had over 700 people working for him on gaseous diffusion. The process involved hundreds of cascades, in which corrosive uranium hexafluoride diffused through gaseous barriers, becoming progressively more enriched at every stage. A major problem was finding proper seals for the pumps, but by far the greatest difficulty lay in constructing an appropriate diffusion barrier. Construction of the huge K-25 gaseous diffusion plant was well under way before a suitable barrier became available in quantity in 1944. As a backup, Urey championed thermal diffusion.