This paper provides conclusive evidence that cosmic rays cause soft errors in commercial dynamic RAM (DRAM) chips at ground level. Cosmic-ray-induced soft errors in electronic components have long been a problem for the designers of satellites and spacecraft, but they have not generally been considered to be an important influence on memory chip soft error rate (SER) in terrestrial environments. In an experiment designed to determine the effect of cosmic radiation on the SER of a sample of DRAM chips at ground level, the SER of a large number of chips was measured at various locations and altitudes around the US: near sea level in Essex Junction, VT; 200 m underground in a Kansas salt mine; at an altitude of 1.6 km in Boulder, CO; and at 3.1 km in Leadville, CO. The results reported here show that even at sea level there is a significant component of the SER that can be attributed to the effects of cosmic rays, and that the magnitude of the effects increases dramatically at higher altitudes. >