The jumps in seismic velocity and density near 410- and 660-km depths have been postulated to mark phase changes in a locally, chemically homogeneous mantle. Seismic estimates of upper mantle discontinuity thickness and lateral variability have varied, posing difficulties for assessing models with chemical and phase transition layering. Observations of underside high-frequency (~1 Hz) P wave reflections from the 410- and 660-km discontinuity that precede the P'P' phase provide unique constraints on the physical properties of these dynamically critical features. We have collected high-quality recordings of P'P' and its precursors from three sources: the California Seismic Network, the Large Aperture Seismic Array in Montana, and the Global Seismic Network. The array and regional network data have great sensitivity to weak P'P' precursors, whereas the global network recordings have good geographical distribution of bounce points. The only consistently observed reflections originate from depths near 410 km and 660 km. The reflector observed near 520-km depth with longer period waves, which has been interpreted as the olivine ¿--γ phase transition, is too small in impedance contrast or too thick to reflect 1-Hz P waves. Data from all three networks indicate that the 1-Hz P waves from the 660 are bigger than those from the 410. Although large variability in P'P' and its precursors' amplitudes prevents strong conclusions, we show that the P'P' precursor amplitudes, travel time dispersion, and frequency content are consistent with a sharp (≤2 km) 660 and a more diffuse 410, consistent with mineral physics predictions.