Copious vortex line creation in a fast (ca. 3 ms) adiabatic expansion of a volume of liquid 4He through the lambda transition has been observed. The maximum line density, inferred from measurements of second sound attenuation, is found to be much larger than that in a comparable expansion from just below the lambda line. The phenomenon was predicted by Zurek who proposed it as a model of cosmic string creation in the false vacuum/true vacuum phase transition of the early universe.