These data are a collection of annotations from audio files from the Belgian Part of the North Sea, as a part of the LifeWatch Broadband Acoustic Network. The code to obtain the features files can be found on: GitHub - lifewatch/unknown_underwater_sounds (https://github.com/lifewatch/unknown_underwater_sounds). The repository contains all of the codes written related to the study of annotated unknown underwater sounds in the Belgian part of the North Sea. The repository also contains scripts to obtain the results of the paper. ... : Acoustic signals, especially those of biological source, remain unexplored in the Belgian part of the North Sea (BPNS). The BPNS, although dominated by anthrophony (sounds from human activities), is expected to be acoustically diverse given the presence of biodiverse sandbanks, gravel beds and artificial hard structures. Under the framework of the LifeWatch Broadband Acoustic Network, sound data have been collected since the spring of 2020. These recordings, encompassing both biophony, geophony and anthrophony, have been listened to and annotated using Raven Pro version 1.6.52 for unknown, acoustically salient sounds. Sounds of known origin which were clearly not biological were excluded. Some examples of annotated sounds from revised clusters were manually picked to define a few groups of distinct sounds in the BPNS. ...