The abrupt increase in seismic velocity and density about 410 km below the surface of the Earth occurs at the depth where a phase change in olivine is expected [Akaogi et al., 1989; Katsura and Ito, 1989; Ringwood, 1969>. We have assembled recordings from 22 short-period seismometers in Washington and British Columbia of 35 nuclear explosions in Nevada. These data reveal relatively noise-free reflections from the ''410'' discontinuity. Most of the transition is completed within a depth interval of less than 10 km under the western United States. This result adds to the accumulating seismic evidence of a fairly sharp ''410'' transition. ¿ American Geophysical Union 1995.