  Video grounding aims to locate a moment of interest matching the given query
sentence from an untrimmed video. Previous works ignore the {sparsity dilemma}
in video annotations, which fails to provide the context information between
potential events and query sentences in the dataset. In this paper, we contend
that exploiting easily available captions which describe general actions, i.e.,
auxiliary captions defined in our paper, will significantly boost the
performance. To this end, we propose an Auxiliary Caption Network (ACNet) for
video grounding. Specifically, we first introduce dense video captioning to
generate dense captions and then obtain auxiliary captions by Non-Auxiliary
Caption Suppression (NACS). To capture the potential information in auxiliary
captions, we propose Caption Guided Attention (CGA) project the semantic
relations between auxiliary captions and query sentences into temporal space
and fuse them into visual representations. Considering the gap between
auxiliary captions and ground truth, we propose Asymmetric Cross-modal
Contrastive Learning (ACCL) for constructing more negative pairs to maximize
cross-modal mutual information. Extensive experiments on three public datasets
(i.e., ActivityNet Captions, TACoS and ActivityNet-CG) demonstrate that our
method significantly outperforms state-of-the-art methods.
