1. A computer-implemented method comprising: identifying, in a plurality of digital videos, a plurality of candidate volumes representing spatio-temporal segments of the digital videos, wherein each of the candidate volumes corresponds to a contiguous sequence of spatial portions of video frames of one of the digital videos, has a starting time and an ending time, and potentially represents a discrete object or action within the video frames, wherein identifying the candidate volumes in the digital videos comprises: stabilizing the digital videos using a video stabilization algorithm and identifying, as a stable segment, a contiguous sequence of frames in one of the digital videos in which a degree of background motion is below a threshold, using a measure of background motion produced by the video stabilization algorithm; determining, for each of the identified candidate volumes, features characterizing the candidate volume, wherein the features are determined from visual properties of the spatial portions of the video frames contained in the candidate volumes; and assigning a verified label to each volume of a plurality of the identified candidate volumes using the determined features, the verified label indicating a particular object or action represented by the volume to which the verified label is assigned.