English: Supplementary Movie 1 Propagating phonons in a WSe2 flake. Fast-traveling, periodic contrast in the WSe2 specimen emanates from both step-edges in the center of the frames and are also seen at the crystal-vacuum interface running from the middle-left to the upper-right. After a few-hundred ps, a slowly-propagating wave can be observed in the center region, which travels from the interface to the step-edge, where it is damped. The images were acquired with a 25-kHz repetition rate and an 18-s integration time per frame. The video illustrates dynamics at 5-ps steps spanning -100 to 1,795 ps (380 total frames) and plays at 40 frames per second (fps) (i.e., dynamics slowed by 5 x 109 times).