In the summer of 1921, a small team of physicists and geologists (William P. Haseman, J. Clarence Karcher, Irving Perrine, and Daniel W. Ohern) performed a historical experiment near the Vines Branch area in south-central Oklahoma. Using a dynamite charge as a seismic source and a special instrument called a seismograph (Figure 1), the team recorded seismic waves that had traveled through the subsurface of the earth. Analysis of the recorded data (Figure 2) showed that seismic reflections from a boundary between two underground rock layers had been detected. Further analysis of the data produced an image of the subsurface—called a seismic reflection profile (Figure 3a)—that agreed with a known geologic feature. That result is widely regarded as the first proof that an accurate image of the earth's subsurface could be made using reflected seismic waves.